1SELECT(7)                PostgreSQL 13.4 Documentation               SELECT(7)
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NAME

6       SELECT, TABLE, WITH - retrieve rows from a table or view
7

SYNOPSIS

9       [ WITH [ RECURSIVE ] with_query [, ...] ]
10       SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
11           [ * | expression [ [ AS ] output_name ] [, ...] ]
12           [ FROM from_item [, ...] ]
13           [ WHERE condition ]
14           [ GROUP BY grouping_element [, ...] ]
15           [ HAVING condition ]
16           [ WINDOW window_name AS ( window_definition ) [, ...] ]
17           [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
18           [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
19           [ LIMIT { count | ALL } ]
20           [ OFFSET start [ ROW | ROWS ] ]
21           [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES } ]
22           [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
23
24       where from_item can be one of:
25
26           [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
27                       [ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ]
28           [ LATERAL ] ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
29           with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
30           [ LATERAL ] function_name ( [ argument [, ...] ] )
31                       [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
32           [ LATERAL ] function_name ( [ argument [, ...] ] ) [ AS ] alias ( column_definition [, ...] )
33           [ LATERAL ] function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
34           [ LATERAL ] ROWS FROM( function_name ( [ argument [, ...] ] ) [ AS ( column_definition [, ...] ) ] [, ...] )
35                       [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
36           from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]
37
38       and grouping_element can be one of:
39
40           ( )
41           expression
42           ( expression [, ...] )
43           ROLLUP ( { expression | ( expression [, ...] ) } [, ...] )
44           CUBE ( { expression | ( expression [, ...] ) } [, ...] )
45           GROUPING SETS ( grouping_element [, ...] )
46
47       and with_query is:
48
49           with_query_name [ ( column_name [, ...] ) ] AS [ [ NOT ] MATERIALIZED ] ( select | values | insert | update | delete )
50
51       TABLE [ ONLY ] table_name [ * ]
52

DESCRIPTION

54       SELECT retrieves rows from zero or more tables. The general processing
55       of SELECT is as follows:
56
57        1. All queries in the WITH list are computed. These effectively serve
58           as temporary tables that can be referenced in the FROM list. A WITH
59           query that is referenced more than once in FROM is computed only
60           once, unless specified otherwise with NOT MATERIALIZED. (See WITH
61           Clause below.)
62
63        2. All elements in the FROM list are computed. (Each element in the
64           FROM list is a real or virtual table.) If more than one element is
65           specified in the FROM list, they are cross-joined together. (See
66           FROM Clause below.)
67
68        3. If the WHERE clause is specified, all rows that do not satisfy the
69           condition are eliminated from the output. (See WHERE Clause below.)
70
71        4. If the GROUP BY clause is specified, or if there are aggregate
72           function calls, the output is combined into groups of rows that
73           match on one or more values, and the results of aggregate functions
74           are computed. If the HAVING clause is present, it eliminates groups
75           that do not satisfy the given condition. (See GROUP BY Clause and
76           HAVING Clause below.)
77
78        5. The actual output rows are computed using the SELECT output
79           expressions for each selected row or row group. (See SELECT List
80           below.)
81
82        6. SELECT DISTINCT eliminates duplicate rows from the result.  SELECT
83           DISTINCT ON eliminates rows that match on all the specified
84           expressions.  SELECT ALL (the default) will return all candidate
85           rows, including duplicates. (See DISTINCT Clause below.)
86
87        7. Using the operators UNION, INTERSECT, and EXCEPT, the output of
88           more than one SELECT statement can be combined to form a single
89           result set. The UNION operator returns all rows that are in one or
90           both of the result sets. The INTERSECT operator returns all rows
91           that are strictly in both result sets. The EXCEPT operator returns
92           the rows that are in the first result set but not in the second. In
93           all three cases, duplicate rows are eliminated unless ALL is
94           specified. The noise word DISTINCT can be added to explicitly
95           specify eliminating duplicate rows. Notice that DISTINCT is the
96           default behavior here, even though ALL is the default for SELECT
97           itself. (See UNION Clause, INTERSECT Clause, and EXCEPT Clause
98           below.)
99
100        8. If the ORDER BY clause is specified, the returned rows are sorted
101           in the specified order. If ORDER BY is not given, the rows are
102           returned in whatever order the system finds fastest to produce.
103           (See ORDER BY Clause below.)
104
105        9. If the LIMIT (or FETCH FIRST) or OFFSET clause is specified, the
106           SELECT statement only returns a subset of the result rows. (See
107           LIMIT Clause below.)
108
109       10. If FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE or FOR KEY SHARE is
110           specified, the SELECT statement locks the selected rows against
111           concurrent updates. (See The Locking Clause below.)
112
113       You must have SELECT privilege on each column used in a SELECT command.
114       The use of FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE or FOR KEY SHARE
115       requires UPDATE privilege as well (for at least one column of each
116       table so selected).
117

PARAMETERS

119   WITH Clause
120       The WITH clause allows you to specify one or more subqueries that can
121       be referenced by name in the primary query. The subqueries effectively
122       act as temporary tables or views for the duration of the primary query.
123       Each subquery can be a SELECT, TABLE, VALUES, INSERT, UPDATE or DELETE
124       statement. When writing a data-modifying statement (INSERT, UPDATE or
125       DELETE) in WITH, it is usual to include a RETURNING clause. It is the
126       output of RETURNING, not the underlying table that the statement
127       modifies, that forms the temporary table that is read by the primary
128       query. If RETURNING is omitted, the statement is still executed, but it
129       produces no output so it cannot be referenced as a table by the primary
130       query.
131
132       A name (without schema qualification) must be specified for each WITH
133       query. Optionally, a list of column names can be specified; if this is
134       omitted, the column names are inferred from the subquery.
135
136       If RECURSIVE is specified, it allows a SELECT subquery to reference
137       itself by name. Such a subquery must have the form
138
139           non_recursive_term UNION [ ALL | DISTINCT ] recursive_term
140
141       where the recursive self-reference must appear on the right-hand side
142       of the UNION. Only one recursive self-reference is permitted per query.
143       Recursive data-modifying statements are not supported, but you can use
144       the results of a recursive SELECT query in a data-modifying statement.
145       See Section 7.8 for an example.
146
147       Another effect of RECURSIVE is that WITH queries need not be ordered: a
148       query can reference another one that is later in the list. (However,
149       circular references, or mutual recursion, are not implemented.) Without
150       RECURSIVE, WITH queries can only reference sibling WITH queries that
151       are earlier in the WITH list.
152
153       When there are multiple queries in the WITH clause, RECURSIVE should be
154       written only once, immediately after WITH. It applies to all queries in
155       the WITH clause, though it has no effect on queries that do not use
156       recursion or forward references.
157
158       The primary query and the WITH queries are all (notionally) executed at
159       the same time. This implies that the effects of a data-modifying
160       statement in WITH cannot be seen from other parts of the query, other
161       than by reading its RETURNING output. If two such data-modifying
162       statements attempt to modify the same row, the results are unspecified.
163
164       A key property of WITH queries is that they are normally evaluated only
165       once per execution of the primary query, even if the primary query
166       refers to them more than once. In particular, data-modifying statements
167       are guaranteed to be executed once and only once, regardless of whether
168       the primary query reads all or any of their output.
169
170       However, a WITH query can be marked NOT MATERIALIZED to remove this
171       guarantee. In that case, the WITH query can be folded into the primary
172       query much as though it were a simple sub-SELECT in the primary query's
173       FROM clause. This results in duplicate computations if the primary
174       query refers to that WITH query more than once; but if each such use
175       requires only a few rows of the WITH query's total output, NOT
176       MATERIALIZED can provide a net savings by allowing the queries to be
177       optimized jointly.  NOT MATERIALIZED is ignored if it is attached to a
178       WITH query that is recursive or is not side-effect-free (i.e., is not a
179       plain SELECT containing no volatile functions).
180
181       By default, a side-effect-free WITH query is folded into the primary
182       query if it is used exactly once in the primary query's FROM clause.
183       This allows joint optimization of the two query levels in situations
184       where that should be semantically invisible. However, such folding can
185       be prevented by marking the WITH query as MATERIALIZED. That might be
186       useful, for example, if the WITH query is being used as an optimization
187       fence to prevent the planner from choosing a bad plan.  PostgreSQL
188       versions before v12 never did such folding, so queries written for
189       older versions might rely on WITH to act as an optimization fence.
190
191       See Section 7.8 for additional information.
192
193   FROM Clause
194       The FROM clause specifies one or more source tables for the SELECT. If
195       multiple sources are specified, the result is the Cartesian product
196       (cross join) of all the sources. But usually qualification conditions
197       are added (via WHERE) to restrict the returned rows to a small subset
198       of the Cartesian product.
199
200       The FROM clause can contain the following elements:
201
202       table_name
203           The name (optionally schema-qualified) of an existing table or
204           view. If ONLY is specified before the table name, only that table
205           is scanned. If ONLY is not specified, the table and all its
206           descendant tables (if any) are scanned. Optionally, * can be
207           specified after the table name to explicitly indicate that
208           descendant tables are included.
209
210       alias
211           A substitute name for the FROM item containing the alias. An alias
212           is used for brevity or to eliminate ambiguity for self-joins (where
213           the same table is scanned multiple times). When an alias is
214           provided, it completely hides the actual name of the table or
215           function; for example given FROM foo AS f, the remainder of the
216           SELECT must refer to this FROM item as f not foo. If an alias is
217           written, a column alias list can also be written to provide
218           substitute names for one or more columns of the table.
219
220       TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed )
221       ]
222           A TABLESAMPLE clause after a table_name indicates that the
223           specified sampling_method should be used to retrieve a subset of
224           the rows in that table. This sampling precedes the application of
225           any other filters such as WHERE clauses. The standard PostgreSQL
226           distribution includes two sampling methods, BERNOULLI and SYSTEM,
227           and other sampling methods can be installed in the database via
228           extensions.
229
230           The BERNOULLI and SYSTEM sampling methods each accept a single
231           argument which is the fraction of the table to sample, expressed as
232           a percentage between 0 and 100. This argument can be any
233           real-valued expression. (Other sampling methods might accept more
234           or different arguments.) These two methods each return a
235           randomly-chosen sample of the table that will contain approximately
236           the specified percentage of the table's rows. The BERNOULLI method
237           scans the whole table and selects or ignores individual rows
238           independently with the specified probability. The SYSTEM method
239           does block-level sampling with each block having the specified
240           chance of being selected; all rows in each selected block are
241           returned. The SYSTEM method is significantly faster than the
242           BERNOULLI method when small sampling percentages are specified, but
243           it may return a less-random sample of the table as a result of
244           clustering effects.
245
246           The optional REPEATABLE clause specifies a seed number or
247           expression to use for generating random numbers within the sampling
248           method. The seed value can be any non-null floating-point value.
249           Two queries that specify the same seed and argument values will
250           select the same sample of the table, if the table has not been
251           changed meanwhile. But different seed values will usually produce
252           different samples. If REPEATABLE is not given then a new random
253           sample is selected for each query, based upon a system-generated
254           seed. Note that some add-on sampling methods do not accept
255           REPEATABLE, and will always produce new samples on each use.
256
257       select
258           A sub-SELECT can appear in the FROM clause. This acts as though its
259           output were created as a temporary table for the duration of this
260           single SELECT command. Note that the sub-SELECT must be surrounded
261           by parentheses, and an alias must be provided for it. A VALUES(7)
262           command can also be used here.
263
264       with_query_name
265           A WITH query is referenced by writing its name, just as though the
266           query's name were a table name. (In fact, the WITH query hides any
267           real table of the same name for the purposes of the primary query.
268           If necessary, you can refer to a real table of the same name by
269           schema-qualifying the table's name.) An alias can be provided in
270           the same way as for a table.
271
272       function_name
273           Function calls can appear in the FROM clause. (This is especially
274           useful for functions that return result sets, but any function can
275           be used.) This acts as though the function's output were created as
276           a temporary table for the duration of this single SELECT command.
277           If the function's result type is composite (including the case of a
278           function with multiple OUT parameters), each attribute becomes a
279           separate column in the implicit table.
280
281           When the optional WITH ORDINALITY clause is added to the function
282           call, an additional column of type bigint will be appended to the
283           function's result column(s). This column numbers the rows of the
284           function's result set, starting from 1. By default, this column is
285           named ordinality.
286
287           An alias can be provided in the same way as for a table. If an
288           alias is written, a column alias list can also be written to
289           provide substitute names for one or more attributes of the
290           function's composite return type, including the ordinality column
291           if present.
292
293           Multiple function calls can be combined into a single FROM-clause
294           item by surrounding them with ROWS FROM( ... ). The output of such
295           an item is the concatenation of the first row from each function,
296           then the second row from each function, etc. If some of the
297           functions produce fewer rows than others, null values are
298           substituted for the missing data, so that the total number of rows
299           returned is always the same as for the function that produced the
300           most rows.
301
302           If the function has been defined as returning the record data type,
303           then an alias or the key word AS must be present, followed by a
304           column definition list in the form ( column_name data_type [, ...
305           ]). The column definition list must match the actual number and
306           types of columns returned by the function.
307
308           When using the ROWS FROM( ... ) syntax, if one of the functions
309           requires a column definition list, it's preferred to put the column
310           definition list after the function call inside ROWS FROM( ... ). A
311           column definition list can be placed after the ROWS FROM( ... )
312           construct only if there's just a single function and no WITH
313           ORDINALITY clause.
314
315           To use ORDINALITY together with a column definition list, you must
316           use the ROWS FROM( ... ) syntax and put the column definition list
317           inside ROWS FROM( ... ).
318
319       join_type
320           One of
321
322           •   [ INNER ] JOIN
323
324           •   LEFT [ OUTER ] JOIN
325
326           •   RIGHT [ OUTER ] JOIN
327
328           •   FULL [ OUTER ] JOIN
329
330           •   CROSS JOIN
331
332           For the INNER and OUTER join types, a join condition must be
333           specified, namely exactly one of NATURAL, ON join_condition, or
334           USING (join_column [, ...]). See below for the meaning. For CROSS
335           JOIN, none of these clauses can appear.
336
337           A JOIN clause combines two FROM items, which for convenience we
338           will refer to as “tables”, though in reality they can be any type
339           of FROM item. Use parentheses if necessary to determine the order
340           of nesting. In the absence of parentheses, JOINs nest
341           left-to-right. In any case JOIN binds more tightly than the commas
342           separating FROM-list items.
343
344           CROSS JOIN and INNER JOIN produce a simple Cartesian product, the
345           same result as you get from listing the two tables at the top level
346           of FROM, but restricted by the join condition (if any).  CROSS JOIN
347           is equivalent to INNER JOIN ON (TRUE), that is, no rows are removed
348           by qualification. These join types are just a notational
349           convenience, since they do nothing you couldn't do with plain FROM
350           and WHERE.
351
352           LEFT OUTER JOIN returns all rows in the qualified Cartesian product
353           (i.e., all combined rows that pass its join condition), plus one
354           copy of each row in the left-hand table for which there was no
355           right-hand row that passed the join condition. This left-hand row
356           is extended to the full width of the joined table by inserting null
357           values for the right-hand columns. Note that only the JOIN clause's
358           own condition is considered while deciding which rows have matches.
359           Outer conditions are applied afterwards.
360
361           Conversely, RIGHT OUTER JOIN returns all the joined rows, plus one
362           row for each unmatched right-hand row (extended with nulls on the
363           left). This is just a notational convenience, since you could
364           convert it to a LEFT OUTER JOIN by switching the left and right
365           tables.
366
367           FULL OUTER JOIN returns all the joined rows, plus one row for each
368           unmatched left-hand row (extended with nulls on the right), plus
369           one row for each unmatched right-hand row (extended with nulls on
370           the left).
371
372       ON join_condition
373           join_condition is an expression resulting in a value of type
374           boolean (similar to a WHERE clause) that specifies which rows in a
375           join are considered to match.
376
377       USING ( join_column [, ...] )
378           A clause of the form USING ( a, b, ... ) is shorthand for ON
379           left_table.a = right_table.a AND left_table.b = right_table.b ....
380           Also, USING implies that only one of each pair of equivalent
381           columns will be included in the join output, not both.
382
383       NATURAL
384           NATURAL is shorthand for a USING list that mentions all columns in
385           the two tables that have matching names. If there are no common
386           column names, NATURAL is equivalent to ON TRUE.
387
388       LATERAL
389           The LATERAL key word can precede a sub-SELECT FROM item. This
390           allows the sub-SELECT to refer to columns of FROM items that appear
391           before it in the FROM list. (Without LATERAL, each sub-SELECT is
392           evaluated independently and so cannot cross-reference any other
393           FROM item.)
394
395           LATERAL can also precede a function-call FROM item, but in this
396           case it is a noise word, because the function expression can refer
397           to earlier FROM items in any case.
398
399           A LATERAL item can appear at top level in the FROM list, or within
400           a JOIN tree. In the latter case it can also refer to any items that
401           are on the left-hand side of a JOIN that it is on the right-hand
402           side of.
403
404           When a FROM item contains LATERAL cross-references, evaluation
405           proceeds as follows: for each row of the FROM item providing the
406           cross-referenced column(s), or set of rows of multiple FROM items
407           providing the columns, the LATERAL item is evaluated using that row
408           or row set's values of the columns. The resulting row(s) are joined
409           as usual with the rows they were computed from. This is repeated
410           for each row or set of rows from the column source table(s).
411
412           The column source table(s) must be INNER or LEFT joined to the
413           LATERAL item, else there would not be a well-defined set of rows
414           from which to compute each set of rows for the LATERAL item. Thus,
415           although a construct such as X RIGHT JOIN LATERAL Y is
416           syntactically valid, it is not actually allowed for Y to reference
417           X.
418
419   WHERE Clause
420       The optional WHERE clause has the general form
421
422           WHERE condition
423
424       where condition is any expression that evaluates to a result of type
425       boolean. Any row that does not satisfy this condition will be
426       eliminated from the output. A row satisfies the condition if it returns
427       true when the actual row values are substituted for any variable
428       references.
429
430   GROUP BY Clause
431       The optional GROUP BY clause has the general form
432
433           GROUP BY grouping_element [, ...]
434
435       GROUP BY will condense into a single row all selected rows that share
436       the same values for the grouped expressions. An expression used inside
437       a grouping_element can be an input column name, or the name or ordinal
438       number of an output column (SELECT list item), or an arbitrary
439       expression formed from input-column values. In case of ambiguity, a
440       GROUP BY name will be interpreted as an input-column name rather than
441       an output column name.
442
443       If any of GROUPING SETS, ROLLUP or CUBE are present as grouping
444       elements, then the GROUP BY clause as a whole defines some number of
445       independent grouping sets. The effect of this is equivalent to
446       constructing a UNION ALL between subqueries with the individual
447       grouping sets as their GROUP BY clauses. For further details on the
448       handling of grouping sets see Section 7.2.4.
449
450       Aggregate functions, if any are used, are computed across all rows
451       making up each group, producing a separate value for each group. (If
452       there are aggregate functions but no GROUP BY clause, the query is
453       treated as having a single group comprising all the selected rows.) The
454       set of rows fed to each aggregate function can be further filtered by
455       attaching a FILTER clause to the aggregate function call; see
456       Section 4.2.7 for more information. When a FILTER clause is present,
457       only those rows matching it are included in the input to that aggregate
458       function.
459
460       When GROUP BY is present, or any aggregate functions are present, it is
461       not valid for the SELECT list expressions to refer to ungrouped columns
462       except within aggregate functions or when the ungrouped column is
463       functionally dependent on the grouped columns, since there would
464       otherwise be more than one possible value to return for an ungrouped
465       column. A functional dependency exists if the grouped columns (or a
466       subset thereof) are the primary key of the table containing the
467       ungrouped column.
468
469       Keep in mind that all aggregate functions are evaluated before
470       evaluating any “scalar” expressions in the HAVING clause or SELECT
471       list. This means that, for example, a CASE expression cannot be used to
472       skip evaluation of an aggregate function; see Section 4.2.14.
473
474       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
475       cannot be specified with GROUP BY.
476
477   HAVING Clause
478       The optional HAVING clause has the general form
479
480           HAVING condition
481
482       where condition is the same as specified for the WHERE clause.
483
484       HAVING eliminates group rows that do not satisfy the condition.  HAVING
485       is different from WHERE: WHERE filters individual rows before the
486       application of GROUP BY, while HAVING filters group rows created by
487       GROUP BY. Each column referenced in condition must unambiguously
488       reference a grouping column, unless the reference appears within an
489       aggregate function or the ungrouped column is functionally dependent on
490       the grouping columns.
491
492       The presence of HAVING turns a query into a grouped query even if there
493       is no GROUP BY clause. This is the same as what happens when the query
494       contains aggregate functions but no GROUP BY clause. All the selected
495       rows are considered to form a single group, and the SELECT list and
496       HAVING clause can only reference table columns from within aggregate
497       functions. Such a query will emit a single row if the HAVING condition
498       is true, zero rows if it is not true.
499
500       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
501       cannot be specified with HAVING.
502
503   WINDOW Clause
504       The optional WINDOW clause has the general form
505
506           WINDOW window_name AS ( window_definition ) [, ...]
507
508       where window_name is a name that can be referenced from OVER clauses or
509       subsequent window definitions, and window_definition is
510
511           [ existing_window_name ]
512           [ PARTITION BY expression [, ...] ]
513           [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
514           [ frame_clause ]
515
516       If an existing_window_name is specified it must refer to an earlier
517       entry in the WINDOW list; the new window copies its partitioning clause
518       from that entry, as well as its ordering clause if any. In this case
519       the new window cannot specify its own PARTITION BY clause, and it can
520       specify ORDER BY only if the copied window does not have one. The new
521       window always uses its own frame clause; the copied window must not
522       specify a frame clause.
523
524       The elements of the PARTITION BY list are interpreted in much the same
525       fashion as elements of a GROUP BY clause, except that they are always
526       simple expressions and never the name or number of an output column.
527       Another difference is that these expressions can contain aggregate
528       function calls, which are not allowed in a regular GROUP BY clause.
529       They are allowed here because windowing occurs after grouping and
530       aggregation.
531
532       Similarly, the elements of the ORDER BY list are interpreted in much
533       the same fashion as elements of a statement-level ORDER BY clause,
534       except that the expressions are always taken as simple expressions and
535       never the name or number of an output column.
536
537       The optional frame_clause defines the window frame for window functions
538       that depend on the frame (not all do). The window frame is a set of
539       related rows for each row of the query (called the current row). The
540       frame_clause can be one of
541
542           { RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
543           { RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]
544
545       where frame_start and frame_end can be one of
546
547           UNBOUNDED PRECEDING
548           offset PRECEDING
549           CURRENT ROW
550           offset FOLLOWING
551           UNBOUNDED FOLLOWING
552
553       and frame_exclusion can be one of
554
555           EXCLUDE CURRENT ROW
556           EXCLUDE GROUP
557           EXCLUDE TIES
558           EXCLUDE NO OTHERS
559
560       If frame_end is omitted it defaults to CURRENT ROW. Restrictions are
561       that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be
562       UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in
563       the above list of frame_start and frame_end options than the
564       frame_start choice does — for example RANGE BETWEEN CURRENT ROW AND
565       offset PRECEDING is not allowed.
566
567       The default framing option is RANGE UNBOUNDED PRECEDING, which is the
568       same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the
569       frame to be all rows from the partition start up through the current
570       row's last peer (a row that the window's ORDER BY clause considers
571       equivalent to the current row; all rows are peers if there is no ORDER
572       BY). In general, UNBOUNDED PRECEDING means that the frame starts with
573       the first row of the partition, and similarly UNBOUNDED FOLLOWING means
574       that the frame ends with the last row of the partition, regardless of
575       RANGE, ROWS or GROUPS mode. In ROWS mode, CURRENT ROW means that the
576       frame starts or ends with the current row; but in RANGE or GROUPS mode
577       it means that the frame starts or ends with the current row's first or
578       last peer in the ORDER BY ordering. The offset PRECEDING and offset
579       FOLLOWING options vary in meaning depending on the frame mode. In ROWS
580       mode, the offset is an integer indicating that the frame starts or ends
581       that many rows before or after the current row. In GROUPS mode, the
582       offset is an integer indicating that the frame starts or ends that many
583       peer groups before or after the current row's peer group, where a peer
584       group is a group of rows that are equivalent according to the window's
585       ORDER BY clause. In RANGE mode, use of an offset option requires that
586       there be exactly one ORDER BY column in the window definition. Then the
587       frame contains those rows whose ordering column value is no more than
588       offset less than (for PRECEDING) or more than (for FOLLOWING) the
589       current row's ordering column value. In these cases the data type of
590       the offset expression depends on the data type of the ordering column.
591       For numeric ordering columns it is typically of the same type as the
592       ordering column, but for datetime ordering columns it is an interval.
593       In all these cases, the value of the offset must be non-null and
594       non-negative. Also, while the offset does not have to be a simple
595       constant, it cannot contain variables, aggregate functions, or window
596       functions.
597
598       The frame_exclusion option allows rows around the current row to be
599       excluded from the frame, even if they would be included according to
600       the frame start and frame end options.  EXCLUDE CURRENT ROW excludes
601       the current row from the frame.  EXCLUDE GROUP excludes the current row
602       and its ordering peers from the frame.  EXCLUDE TIES excludes any peers
603       of the current row from the frame, but not the current row itself.
604       EXCLUDE NO OTHERS simply specifies explicitly the default behavior of
605       not excluding the current row or its peers.
606
607       Beware that the ROWS mode can produce unpredictable results if the
608       ORDER BY ordering does not order the rows uniquely. The RANGE and
609       GROUPS modes are designed to ensure that rows that are peers in the
610       ORDER BY ordering are treated alike: all rows of a given peer group
611       will be in the frame or excluded from it.
612
613       The purpose of a WINDOW clause is to specify the behavior of window
614       functions appearing in the query's SELECT list or ORDER BY clause.
615       These functions can reference the WINDOW clause entries by name in
616       their OVER clauses. A WINDOW clause entry does not have to be
617       referenced anywhere, however; if it is not used in the query it is
618       simply ignored. It is possible to use window functions without any
619       WINDOW clause at all, since a window function call can specify its
620       window definition directly in its OVER clause. However, the WINDOW
621       clause saves typing when the same window definition is needed for more
622       than one window function.
623
624       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
625       cannot be specified with WINDOW.
626
627       Window functions are described in detail in Section 3.5, Section 4.2.8,
628       and Section 7.2.5.
629
630   SELECT List
631       The SELECT list (between the key words SELECT and FROM) specifies
632       expressions that form the output rows of the SELECT statement. The
633       expressions can (and usually do) refer to columns computed in the FROM
634       clause.
635
636       Just as in a table, every output column of a SELECT has a name. In a
637       simple SELECT this name is just used to label the column for display,
638       but when the SELECT is a sub-query of a larger query, the name is seen
639       by the larger query as the column name of the virtual table produced by
640       the sub-query. To specify the name to use for an output column, write
641       AS output_name after the column's expression. (You can omit AS, but
642       only if the desired output name does not match any PostgreSQL keyword
643       (see Appendix C). For protection against possible future keyword
644       additions, it is recommended that you always either write AS or
645       double-quote the output name.) If you do not specify a column name, a
646       name is chosen automatically by PostgreSQL. If the column's expression
647       is a simple column reference then the chosen name is the same as that
648       column's name. In more complex cases a function or type name may be
649       used, or the system may fall back on a generated name such as ?column?.
650
651       An output column's name can be used to refer to the column's value in
652       ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses;
653       there you must write out the expression instead.
654
655       Instead of an expression, * can be written in the output list as a
656       shorthand for all the columns of the selected rows. Also, you can write
657       table_name.*  as a shorthand for the columns coming from just that
658       table. In these cases it is not possible to specify new names with AS;
659       the output column names will be the same as the table columns' names.
660
661       According to the SQL standard, the expressions in the output list
662       should be computed before applying DISTINCT, ORDER BY, or LIMIT. This
663       is obviously necessary when using DISTINCT, since otherwise it's not
664       clear what values are being made distinct. However, in many cases it is
665       convenient if output expressions are computed after ORDER BY and LIMIT;
666       particularly if the output list contains any volatile or expensive
667       functions. With that behavior, the order of function evaluations is
668       more intuitive and there will not be evaluations corresponding to rows
669       that never appear in the output.  PostgreSQL will effectively evaluate
670       output expressions after sorting and limiting, so long as those
671       expressions are not referenced in DISTINCT, ORDER BY or GROUP BY. (As a
672       counterexample, SELECT f(x) FROM tab ORDER BY 1 clearly must evaluate
673       f(x) before sorting.) Output expressions that contain set-returning
674       functions are effectively evaluated after sorting and before limiting,
675       so that LIMIT will act to cut off the output from a set-returning
676       function.
677
678           Note
679           PostgreSQL versions before 9.6 did not provide any guarantees about
680           the timing of evaluation of output expressions versus sorting and
681           limiting; it depended on the form of the chosen query plan.
682
683   DISTINCT Clause
684       If SELECT DISTINCT is specified, all duplicate rows are removed from
685       the result set (one row is kept from each group of duplicates).  SELECT
686       ALL specifies the opposite: all rows are kept; that is the default.
687
688       SELECT DISTINCT ON ( expression [, ...] ) keeps only the first row of
689       each set of rows where the given expressions evaluate to equal. The
690       DISTINCT ON expressions are interpreted using the same rules as for
691       ORDER BY (see above). Note that the “first row” of each set is
692       unpredictable unless ORDER BY is used to ensure that the desired row
693       appears first. For example:
694
695           SELECT DISTINCT ON (location) location, time, report
696               FROM weather_reports
697               ORDER BY location, time DESC;
698
699       retrieves the most recent weather report for each location. But if we
700       had not used ORDER BY to force descending order of time values for each
701       location, we'd have gotten a report from an unpredictable time for each
702       location.
703
704       The DISTINCT ON expression(s) must match the leftmost ORDER BY
705       expression(s). The ORDER BY clause will normally contain additional
706       expression(s) that determine the desired precedence of rows within each
707       DISTINCT ON group.
708
709       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
710       cannot be specified with DISTINCT.
711
712   UNION Clause
713       The UNION clause has this general form:
714
715           select_statement UNION [ ALL | DISTINCT ] select_statement
716
717       select_statement is any SELECT statement without an ORDER BY, LIMIT,
718       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
719       (ORDER BY and LIMIT can be attached to a subexpression if it is
720       enclosed in parentheses. Without parentheses, these clauses will be
721       taken to apply to the result of the UNION, not to its right-hand input
722       expression.)
723
724       The UNION operator computes the set union of the rows returned by the
725       involved SELECT statements. A row is in the set union of two result
726       sets if it appears in at least one of the result sets. The two SELECT
727       statements that represent the direct operands of the UNION must produce
728       the same number of columns, and corresponding columns must be of
729       compatible data types.
730
731       The result of UNION does not contain any duplicate rows unless the ALL
732       option is specified.  ALL prevents elimination of duplicates.
733       (Therefore, UNION ALL is usually significantly quicker than UNION; use
734       ALL when you can.)  DISTINCT can be written to explicitly specify the
735       default behavior of eliminating duplicate rows.
736
737       Multiple UNION operators in the same SELECT statement are evaluated
738       left to right, unless otherwise indicated by parentheses.
739
740       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
741       cannot be specified either for a UNION result or for any input of a
742       UNION.
743
744   INTERSECT Clause
745       The INTERSECT clause has this general form:
746
747           select_statement INTERSECT [ ALL | DISTINCT ] select_statement
748
749       select_statement is any SELECT statement without an ORDER BY, LIMIT,
750       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
751
752       The INTERSECT operator computes the set intersection of the rows
753       returned by the involved SELECT statements. A row is in the
754       intersection of two result sets if it appears in both result sets.
755
756       The result of INTERSECT does not contain any duplicate rows unless the
757       ALL option is specified. With ALL, a row that has m duplicates in the
758       left table and n duplicates in the right table will appear min(m,n)
759       times in the result set.  DISTINCT can be written to explicitly specify
760       the default behavior of eliminating duplicate rows.
761
762       Multiple INTERSECT operators in the same SELECT statement are evaluated
763       left to right, unless parentheses dictate otherwise.  INTERSECT binds
764       more tightly than UNION. That is, A UNION B INTERSECT C will be read as
765       A UNION (B INTERSECT C).
766
767       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
768       cannot be specified either for an INTERSECT result or for any input of
769       an INTERSECT.
770
771   EXCEPT Clause
772       The EXCEPT clause has this general form:
773
774           select_statement EXCEPT [ ALL | DISTINCT ] select_statement
775
776       select_statement is any SELECT statement without an ORDER BY, LIMIT,
777       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
778
779       The EXCEPT operator computes the set of rows that are in the result of
780       the left SELECT statement but not in the result of the right one.
781
782       The result of EXCEPT does not contain any duplicate rows unless the ALL
783       option is specified. With ALL, a row that has m duplicates in the left
784       table and n duplicates in the right table will appear max(m-n,0) times
785       in the result set.  DISTINCT can be written to explicitly specify the
786       default behavior of eliminating duplicate rows.
787
788       Multiple EXCEPT operators in the same SELECT statement are evaluated
789       left to right, unless parentheses dictate otherwise.  EXCEPT binds at
790       the same level as UNION.
791
792       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
793       cannot be specified either for an EXCEPT result or for any input of an
794       EXCEPT.
795
796   ORDER BY Clause
797       The optional ORDER BY clause has this general form:
798
799           ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]
800
801       The ORDER BY clause causes the result rows to be sorted according to
802       the specified expression(s). If two rows are equal according to the
803       leftmost expression, they are compared according to the next expression
804       and so on. If they are equal according to all specified expressions,
805       they are returned in an implementation-dependent order.
806
807       Each expression can be the name or ordinal number of an output column
808       (SELECT list item), or it can be an arbitrary expression formed from
809       input-column values.
810
811       The ordinal number refers to the ordinal (left-to-right) position of
812       the output column. This feature makes it possible to define an ordering
813       on the basis of a column that does not have a unique name. This is
814       never absolutely necessary because it is always possible to assign a
815       name to an output column using the AS clause.
816
817       It is also possible to use arbitrary expressions in the ORDER BY
818       clause, including columns that do not appear in the SELECT output list.
819       Thus the following statement is valid:
820
821           SELECT name FROM distributors ORDER BY code;
822
823       A limitation of this feature is that an ORDER BY clause applying to the
824       result of a UNION, INTERSECT, or EXCEPT clause can only specify an
825       output column name or number, not an expression.
826
827       If an ORDER BY expression is a simple name that matches both an output
828       column name and an input column name, ORDER BY will interpret it as the
829       output column name. This is the opposite of the choice that GROUP BY
830       will make in the same situation. This inconsistency is made to be
831       compatible with the SQL standard.
832
833       Optionally one can add the key word ASC (ascending) or DESC
834       (descending) after any expression in the ORDER BY clause. If not
835       specified, ASC is assumed by default. Alternatively, a specific
836       ordering operator name can be specified in the USING clause. An
837       ordering operator must be a less-than or greater-than member of some
838       B-tree operator family.  ASC is usually equivalent to USING < and DESC
839       is usually equivalent to USING >. (But the creator of a user-defined
840       data type can define exactly what the default sort ordering is, and it
841       might correspond to operators with other names.)
842
843       If NULLS LAST is specified, null values sort after all non-null values;
844       if NULLS FIRST is specified, null values sort before all non-null
845       values. If neither is specified, the default behavior is NULLS LAST
846       when ASC is specified or implied, and NULLS FIRST when DESC is
847       specified (thus, the default is to act as though nulls are larger than
848       non-nulls). When USING is specified, the default nulls ordering depends
849       on whether the operator is a less-than or greater-than operator.
850
851       Note that ordering options apply only to the expression they follow;
852       for example ORDER BY x, y DESC does not mean the same thing as ORDER BY
853       x DESC, y DESC.
854
855       Character-string data is sorted according to the collation that applies
856       to the column being sorted. That can be overridden at need by including
857       a COLLATE clause in the expression, for example ORDER BY mycolumn
858       COLLATE "en_US". For more information see Section 4.2.10 and
859       Section 23.2.
860
861   LIMIT Clause
862       The LIMIT clause consists of two independent sub-clauses:
863
864           LIMIT { count | ALL }
865           OFFSET start
866
867       The parameter count specifies the maximum number of rows to return,
868       while start specifies the number of rows to skip before starting to
869       return rows. When both are specified, start rows are skipped before
870       starting to count the count rows to be returned.
871
872       If the count expression evaluates to NULL, it is treated as LIMIT ALL,
873       i.e., no limit. If start evaluates to NULL, it is treated the same as
874       OFFSET 0.
875
876       SQL:2008 introduced a different syntax to achieve the same result,
877       which PostgreSQL also supports. It is:
878
879           OFFSET start { ROW | ROWS }
880           FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }
881
882       In this syntax, the start or count value is required by the standard to
883       be a literal constant, a parameter, or a variable name; as a PostgreSQL
884       extension, other expressions are allowed, but will generally need to be
885       enclosed in parentheses to avoid ambiguity. If count is omitted in a
886       FETCH clause, it defaults to 1. The WITH TIES option is used to return
887       any additional rows that tie for the last place in the result set
888       according to the ORDER BY clause; ORDER BY is mandatory in this case.
889       ROW and ROWS as well as FIRST and NEXT are noise words that don't
890       influence the effects of these clauses. According to the standard, the
891       OFFSET clause must come before the FETCH clause if both are present;
892       but PostgreSQL is laxer and allows either order.
893
894       When using LIMIT, it is a good idea to use an ORDER BY clause that
895       constrains the result rows into a unique order. Otherwise you will get
896       an unpredictable subset of the query's rows — you might be asking for
897       the tenth through twentieth rows, but tenth through twentieth in what
898       ordering? You don't know what ordering unless you specify ORDER BY.
899
900       The query planner takes LIMIT into account when generating a query
901       plan, so you are very likely to get different plans (yielding different
902       row orders) depending on what you use for LIMIT and OFFSET. Thus, using
903       different LIMIT/OFFSET values to select different subsets of a query
904       result will give inconsistent results unless you enforce a predictable
905       result ordering with ORDER BY. This is not a bug; it is an inherent
906       consequence of the fact that SQL does not promise to deliver the
907       results of a query in any particular order unless ORDER BY is used to
908       constrain the order.
909
910       It is even possible for repeated executions of the same LIMIT query to
911       return different subsets of the rows of a table, if there is not an
912       ORDER BY to enforce selection of a deterministic subset. Again, this is
913       not a bug; determinism of the results is simply not guaranteed in such
914       a case.
915
916   The Locking Clause
917       FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE are locking
918       clauses; they affect how SELECT locks rows as they are obtained from
919       the table.
920
921       The locking clause has the general form
922
923           FOR lock_strength [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ]
924
925       where lock_strength can be one of
926
927           UPDATE
928           NO KEY UPDATE
929           SHARE
930           KEY SHARE
931
932       For more information on each row-level lock mode, refer to
933       Section 13.3.2.
934
935       To prevent the operation from waiting for other transactions to commit,
936       use either the NOWAIT or SKIP LOCKED option. With NOWAIT, the statement
937       reports an error, rather than waiting, if a selected row cannot be
938       locked immediately. With SKIP LOCKED, any selected rows that cannot be
939       immediately locked are skipped. Skipping locked rows provides an
940       inconsistent view of the data, so this is not suitable for general
941       purpose work, but can be used to avoid lock contention with multiple
942       consumers accessing a queue-like table. Note that NOWAIT and SKIP
943       LOCKED apply only to the row-level lock(s) — the required ROW SHARE
944       table-level lock is still taken in the ordinary way (see Chapter 13).
945       You can use LOCK(7) with the NOWAIT option first, if you need to
946       acquire the table-level lock without waiting.
947
948       If specific tables are named in a locking clause, then only rows coming
949       from those tables are locked; any other tables used in the SELECT are
950       simply read as usual. A locking clause without a table list affects all
951       tables used in the statement. If a locking clause is applied to a view
952       or sub-query, it affects all tables used in the view or sub-query.
953       However, these clauses do not apply to WITH queries referenced by the
954       primary query. If you want row locking to occur within a WITH query,
955       specify a locking clause within the WITH query.
956
957       Multiple locking clauses can be written if it is necessary to specify
958       different locking behavior for different tables. If the same table is
959       mentioned (or implicitly affected) by more than one locking clause,
960       then it is processed as if it was only specified by the strongest one.
961       Similarly, a table is processed as NOWAIT if that is specified in any
962       of the clauses affecting it. Otherwise, it is processed as SKIP LOCKED
963       if that is specified in any of the clauses affecting it.
964
965       The locking clauses cannot be used in contexts where returned rows
966       cannot be clearly identified with individual table rows; for example
967       they cannot be used with aggregation.
968
969       When a locking clause appears at the top level of a SELECT query, the
970       rows that are locked are exactly those that are returned by the query;
971       in the case of a join query, the rows locked are those that contribute
972       to returned join rows. In addition, rows that satisfied the query
973       conditions as of the query snapshot will be locked, although they will
974       not be returned if they were updated after the snapshot and no longer
975       satisfy the query conditions. If a LIMIT is used, locking stops once
976       enough rows have been returned to satisfy the limit (but note that rows
977       skipped over by OFFSET will get locked). Similarly, if a locking clause
978       is used in a cursor's query, only rows actually fetched or stepped past
979       by the cursor will be locked.
980
981       When a locking clause appears in a sub-SELECT, the rows locked are
982       those returned to the outer query by the sub-query. This might involve
983       fewer rows than inspection of the sub-query alone would suggest, since
984       conditions from the outer query might be used to optimize execution of
985       the sub-query. For example,
986
987           SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
988
989       will lock only rows having col1 = 5, even though that condition is not
990       textually within the sub-query.
991
992       Previous releases failed to preserve a lock which is upgraded by a
993       later savepoint. For example, this code:
994
995           BEGIN;
996           SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
997           SAVEPOINT s;
998           UPDATE mytable SET ... WHERE key = 1;
999           ROLLBACK TO s;
1000
1001       would fail to preserve the FOR UPDATE lock after the ROLLBACK TO. This
1002       has been fixed in release 9.3.
1003
1004           Caution
1005           It is possible for a SELECT command running at the READ COMMITTED
1006           transaction isolation level and using ORDER BY and a locking clause
1007           to return rows out of order. This is because ORDER BY is applied
1008           first. The command sorts the result, but might then block trying to
1009           obtain a lock on one or more of the rows. Once the SELECT unblocks,
1010           some of the ordering column values might have been modified,
1011           leading to those rows appearing to be out of order (though they are
1012           in order in terms of the original column values). This can be
1013           worked around at need by placing the FOR UPDATE/SHARE clause in a
1014           sub-query, for example
1015
1016               SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
1017
1018           Note that this will result in locking all rows of mytable, whereas
1019           FOR UPDATE at the top level would lock only the actually returned
1020           rows. This can make for a significant performance difference,
1021           particularly if the ORDER BY is combined with LIMIT or other
1022           restrictions. So this technique is recommended only if concurrent
1023           updates of the ordering columns are expected and a strictly sorted
1024           result is required.
1025
1026           At the REPEATABLE READ or SERIALIZABLE transaction isolation level
1027           this would cause a serialization failure (with a SQLSTATE of
1028           '40001'), so there is no possibility of receiving rows out of order
1029           under these isolation levels.
1030
1031   TABLE Command
1032       The command
1033
1034           TABLE name
1035
1036       is equivalent to
1037
1038           SELECT * FROM name
1039
1040       It can be used as a top-level command or as a space-saving syntax
1041       variant in parts of complex queries. Only the WITH, UNION, INTERSECT,
1042       EXCEPT, ORDER BY, LIMIT, OFFSET, FETCH and FOR locking clauses can be
1043       used with TABLE; the WHERE clause and any form of aggregation cannot be
1044       used.
1045

EXAMPLES

1047       To join the table films with the table distributors:
1048
1049           SELECT f.title, f.did, d.name, f.date_prod, f.kind
1050               FROM distributors d, films f
1051               WHERE f.did = d.did
1052
1053                  title       | did |     name     | date_prod  |   kind
1054           -------------------+-----+--------------+------------+----------
1055            The Third Man     | 101 | British Lion | 1949-12-23 | Drama
1056            The African Queen | 101 | British Lion | 1951-08-11 | Romantic
1057            ...
1058
1059       To sum the column len of all films and group the results by kind:
1060
1061           SELECT kind, sum(len) AS total FROM films GROUP BY kind;
1062
1063              kind   | total
1064           ----------+-------
1065            Action   | 07:34
1066            Comedy   | 02:58
1067            Drama    | 14:28
1068            Musical  | 06:42
1069            Romantic | 04:38
1070
1071       To sum the column len of all films, group the results by kind and show
1072       those group totals that are less than 5 hours:
1073
1074           SELECT kind, sum(len) AS total
1075               FROM films
1076               GROUP BY kind
1077               HAVING sum(len) < interval '5 hours';
1078
1079              kind   | total
1080           ----------+-------
1081            Comedy   | 02:58
1082            Romantic | 04:38
1083
1084       The following two examples are identical ways of sorting the individual
1085       results according to the contents of the second column (name):
1086
1087           SELECT * FROM distributors ORDER BY name;
1088           SELECT * FROM distributors ORDER BY 2;
1089
1090            did |       name
1091           -----+------------------
1092            109 | 20th Century Fox
1093            110 | Bavaria Atelier
1094            101 | British Lion
1095            107 | Columbia
1096            102 | Jean Luc Godard
1097            113 | Luso films
1098            104 | Mosfilm
1099            103 | Paramount
1100            106 | Toho
1101            105 | United Artists
1102            111 | Walt Disney
1103            112 | Warner Bros.
1104            108 | Westward
1105
1106       The next example shows how to obtain the union of the tables
1107       distributors and actors, restricting the results to those that begin
1108       with the letter W in each table. Only distinct rows are wanted, so the
1109       key word ALL is omitted.
1110
1111           distributors:               actors:
1112            did |     name              id |     name
1113           -----+--------------        ----+----------------
1114            108 | Westward               1 | Woody Allen
1115            111 | Walt Disney            2 | Warren Beatty
1116            112 | Warner Bros.           3 | Walter Matthau
1117            ...                         ...
1118
1119           SELECT distributors.name
1120               FROM distributors
1121               WHERE distributors.name LIKE 'W%'
1122           UNION
1123           SELECT actors.name
1124               FROM actors
1125               WHERE actors.name LIKE 'W%';
1126
1127                 name
1128           ----------------
1129            Walt Disney
1130            Walter Matthau
1131            Warner Bros.
1132            Warren Beatty
1133            Westward
1134            Woody Allen
1135
1136       This example shows how to use a function in the FROM clause, both with
1137       and without a column definition list:
1138
1139           CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
1140               SELECT * FROM distributors WHERE did = $1;
1141           $$ LANGUAGE SQL;
1142
1143           SELECT * FROM distributors(111);
1144            did |    name
1145           -----+-------------
1146            111 | Walt Disney
1147
1148           CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
1149               SELECT * FROM distributors WHERE did = $1;
1150           $$ LANGUAGE SQL;
1151
1152           SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
1153            f1  |     f2
1154           -----+-------------
1155            111 | Walt Disney
1156
1157       Here is an example of a function with an ordinality column added:
1158
1159           SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY;
1160            unnest | ordinality
1161           --------+----------
1162            a      |        1
1163            b      |        2
1164            c      |        3
1165            d      |        4
1166            e      |        5
1167            f      |        6
1168           (6 rows)
1169
1170       This example shows how to use a simple WITH clause:
1171
1172           WITH t AS (
1173               SELECT random() as x FROM generate_series(1, 3)
1174             )
1175           SELECT * FROM t
1176           UNION ALL
1177           SELECT * FROM t
1178
1179                    x
1180           --------------------
1181             0.534150459803641
1182             0.520092216785997
1183            0.0735620250925422
1184             0.534150459803641
1185             0.520092216785997
1186            0.0735620250925422
1187
1188       Notice that the WITH query was evaluated only once, so that we got two
1189       sets of the same three random values.
1190
1191       This example uses WITH RECURSIVE to find all subordinates (direct or
1192       indirect) of the employee Mary, and their level of indirectness, from a
1193       table that shows only direct subordinates:
1194
1195           WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
1196               SELECT 1, employee_name, manager_name
1197               FROM employee
1198               WHERE manager_name = 'Mary'
1199             UNION ALL
1200               SELECT er.distance + 1, e.employee_name, e.manager_name
1201               FROM employee_recursive er, employee e
1202               WHERE er.employee_name = e.manager_name
1203             )
1204           SELECT distance, employee_name FROM employee_recursive;
1205
1206       Notice the typical form of recursive queries: an initial condition,
1207       followed by UNION, followed by the recursive part of the query. Be sure
1208       that the recursive part of the query will eventually return no tuples,
1209       or else the query will loop indefinitely. (See Section 7.8 for more
1210       examples.)
1211
1212       This example uses LATERAL to apply a set-returning function
1213       get_product_names() for each row of the manufacturers table:
1214
1215           SELECT m.name AS mname, pname
1216           FROM manufacturers m, LATERAL get_product_names(m.id) pname;
1217
1218       Manufacturers not currently having any products would not appear in the
1219       result, since it is an inner join. If we wished to include the names of
1220       such manufacturers in the result, we could do:
1221
1222           SELECT m.name AS mname, pname
1223           FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;
1224

COMPATIBILITY

1226       Of course, the SELECT statement is compatible with the SQL standard.
1227       But there are some extensions and some missing features.
1228
1229   Omitted FROM Clauses
1230       PostgreSQL allows one to omit the FROM clause. It has a straightforward
1231       use to compute the results of simple expressions:
1232
1233           SELECT 2+2;
1234
1235            ?column?
1236           ----------
1237                   4
1238
1239       Some other SQL databases cannot do this except by introducing a dummy
1240       one-row table from which to do the SELECT.
1241
1242       Note that if a FROM clause is not specified, the query cannot reference
1243       any database tables. For example, the following query is invalid:
1244
1245           SELECT distributors.* WHERE distributors.name = 'Westward';
1246
1247       PostgreSQL releases prior to 8.1 would accept queries of this form, and
1248       add an implicit entry to the query's FROM clause for each table
1249       referenced by the query. This is no longer allowed.
1250
1251   Empty SELECT Lists
1252       The list of output expressions after SELECT can be empty, producing a
1253       zero-column result table. This is not valid syntax according to the SQL
1254       standard.  PostgreSQL allows it to be consistent with allowing
1255       zero-column tables. However, an empty list is not allowed when DISTINCT
1256       is used.
1257
1258   Omitting the AS Key Word
1259       In the SQL standard, the optional key word AS can be omitted before an
1260       output column name whenever the new column name is a valid column name
1261       (that is, not the same as any reserved keyword).  PostgreSQL is
1262       slightly more restrictive: AS is required if the new column name
1263       matches any keyword at all, reserved or not. Recommended practice is to
1264       use AS or double-quote output column names, to prevent any possible
1265       conflict against future keyword additions.
1266
1267       In FROM items, both the standard and PostgreSQL allow AS to be omitted
1268       before an alias that is an unreserved keyword. But this is impractical
1269       for output column names, because of syntactic ambiguities.
1270
1271   ONLY and Inheritance
1272       The SQL standard requires parentheses around the table name when
1273       writing ONLY, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE
1274       ....  PostgreSQL considers these parentheses to be optional.
1275
1276       PostgreSQL allows a trailing * to be written to explicitly specify the
1277       non-ONLY behavior of including child tables. The standard does not
1278       allow this.
1279
1280       (These points apply equally to all SQL commands supporting the ONLY
1281       option.)
1282
1283   TABLESAMPLE Clause Restrictions
1284       The TABLESAMPLE clause is currently accepted only on regular tables and
1285       materialized views. According to the SQL standard it should be possible
1286       to apply it to any FROM item.
1287
1288   Function Calls in FROM
1289       PostgreSQL allows a function call to be written directly as a member of
1290       the FROM list. In the SQL standard it would be necessary to wrap such a
1291       function call in a sub-SELECT; that is, the syntax FROM func(...) alias
1292       is approximately equivalent to FROM LATERAL (SELECT func(...)) alias.
1293       Note that LATERAL is considered to be implicit; this is because the
1294       standard requires LATERAL semantics for an UNNEST() item in FROM.
1295       PostgreSQL treats UNNEST() the same as other set-returning functions.
1296
1297   Namespace Available to GROUP BY and ORDER BY
1298       In the SQL-92 standard, an ORDER BY clause can only use output column
1299       names or numbers, while a GROUP BY clause can only use expressions
1300       based on input column names.  PostgreSQL extends each of these clauses
1301       to allow the other choice as well (but it uses the standard's
1302       interpretation if there is ambiguity).  PostgreSQL also allows both
1303       clauses to specify arbitrary expressions. Note that names appearing in
1304       an expression will always be taken as input-column names, not as
1305       output-column names.
1306
1307       SQL:1999 and later use a slightly different definition which is not
1308       entirely upward compatible with SQL-92. In most cases, however,
1309       PostgreSQL will interpret an ORDER BY or GROUP BY expression the same
1310       way SQL:1999 does.
1311
1312   Functional Dependencies
1313       PostgreSQL recognizes functional dependency (allowing columns to be
1314       omitted from GROUP BY) only when a table's primary key is included in
1315       the GROUP BY list. The SQL standard specifies additional conditions
1316       that should be recognized.
1317
1318   LIMIT and OFFSET
1319       The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used
1320       by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ...
1321       FETCH {FIRST|NEXT} ...  for the same functionality, as shown above in
1322       LIMIT Clause. This syntax is also used by IBM DB2. (Applications
1323       written for Oracle frequently use a workaround involving the
1324       automatically generated rownum column, which is not available in
1325       PostgreSQL, to implement the effects of these clauses.)
1326
1327   FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, FOR KEY SHARE
1328       Although FOR UPDATE appears in the SQL standard, the standard allows it
1329       only as an option of DECLARE CURSOR.  PostgreSQL allows it in any
1330       SELECT query as well as in sub-SELECTs, but this is an extension. The
1331       FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE variants, as well as the
1332       NOWAIT and SKIP LOCKED options, do not appear in the standard.
1333
1334   Data-Modifying Statements in WITH
1335       PostgreSQL allows INSERT, UPDATE, and DELETE to be used as WITH
1336       queries. This is not found in the SQL standard.
1337
1338   Nonstandard Clauses
1339       DISTINCT ON ( ... ) is an extension of the SQL standard.
1340
1341       ROWS FROM( ... ) is an extension of the SQL standard.
1342
1343       The MATERIALIZED and NOT MATERIALIZED options of WITH are extensions of
1344       the SQL standard.
1345
1346
1347
1348PostgreSQL 13.4                      2021                            SELECT(7)
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