1SELECT(7)                PostgreSQL 12.6 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 ]
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           When the optional WITH ORDINALITY clause is added to the function
278           call, a new column is appended after all the function's output
279           columns with numbering for each row.
280
281           An alias can be provided in the same way as for a table. If an
282           alias is written, a column alias list can also be written to
283           provide substitute names for one or more attributes of the
284           function's composite return type, including the column added by
285           ORDINALITY if present.
286
287           Multiple function calls can be combined into a single FROM-clause
288           item by surrounding them with ROWS FROM( ... ). The output of such
289           an item is the concatenation of the first row from each function,
290           then the second row from each function, etc. If some of the
291           functions produce fewer rows than others, null values are
292           substituted for the missing data, so that the total number of rows
293           returned is always the same as for the function that produced the
294           most rows.
295
296           If the function has been defined as returning the record data type,
297           then an alias or the key word AS must be present, followed by a
298           column definition list in the form ( column_name data_type [, ...
299           ]). The column definition list must match the actual number and
300           types of columns returned by the function.
301
302           When using the ROWS FROM( ... ) syntax, if one of the functions
303           requires a column definition list, it's preferred to put the column
304           definition list after the function call inside ROWS FROM( ... ). A
305           column definition list can be placed after the ROWS FROM( ... )
306           construct only if there's just a single function and no WITH
307           ORDINALITY clause.
308
309           To use ORDINALITY together with a column definition list, you must
310           use the ROWS FROM( ... ) syntax and put the column definition list
311           inside ROWS FROM( ... ).
312
313       join_type
314           One of
315
316           ·   [ INNER ] JOIN
317
318           ·   LEFT [ OUTER ] JOIN
319
320           ·   RIGHT [ OUTER ] JOIN
321
322           ·   FULL [ OUTER ] JOIN
323
324           ·   CROSS JOIN
325
326           For the INNER and OUTER join types, a join condition must be
327           specified, namely exactly one of NATURAL, ON join_condition, or
328           USING (join_column [, ...]). See below for the meaning. For CROSS
329           JOIN, none of these clauses can appear.
330
331           A JOIN clause combines two FROM items, which for convenience we
332           will refer to as “tables”, though in reality they can be any type
333           of FROM item. Use parentheses if necessary to determine the order
334           of nesting. In the absence of parentheses, JOINs nest
335           left-to-right. In any case JOIN binds more tightly than the commas
336           separating FROM-list items.
337
338           CROSS JOIN and INNER JOIN produce a simple Cartesian product, the
339           same result as you get from listing the two tables at the top level
340           of FROM, but restricted by the join condition (if any).  CROSS JOIN
341           is equivalent to INNER JOIN ON (TRUE), that is, no rows are removed
342           by qualification. These join types are just a notational
343           convenience, since they do nothing you couldn't do with plain FROM
344           and WHERE.
345
346           LEFT OUTER JOIN returns all rows in the qualified Cartesian product
347           (i.e., all combined rows that pass its join condition), plus one
348           copy of each row in the left-hand table for which there was no
349           right-hand row that passed the join condition. This left-hand row
350           is extended to the full width of the joined table by inserting null
351           values for the right-hand columns. Note that only the JOIN clause's
352           own condition is considered while deciding which rows have matches.
353           Outer conditions are applied afterwards.
354
355           Conversely, RIGHT OUTER JOIN returns all the joined rows, plus one
356           row for each unmatched right-hand row (extended with nulls on the
357           left). This is just a notational convenience, since you could
358           convert it to a LEFT OUTER JOIN by switching the left and right
359           tables.
360
361           FULL OUTER JOIN returns all the joined rows, plus one row for each
362           unmatched left-hand row (extended with nulls on the right), plus
363           one row for each unmatched right-hand row (extended with nulls on
364           the left).
365
366       ON join_condition
367           join_condition is an expression resulting in a value of type
368           boolean (similar to a WHERE clause) that specifies which rows in a
369           join are considered to match.
370
371       USING ( join_column [, ...] )
372           A clause of the form USING ( a, b, ... ) is shorthand for ON
373           left_table.a = right_table.a AND left_table.b = right_table.b ....
374           Also, USING implies that only one of each pair of equivalent
375           columns will be included in the join output, not both.
376
377       NATURAL
378           NATURAL is shorthand for a USING list that mentions all columns in
379           the two tables that have matching names. If there are no common
380           column names, NATURAL is equivalent to ON TRUE.
381
382       LATERAL
383           The LATERAL key word can precede a sub-SELECT FROM item. This
384           allows the sub-SELECT to refer to columns of FROM items that appear
385           before it in the FROM list. (Without LATERAL, each sub-SELECT is
386           evaluated independently and so cannot cross-reference any other
387           FROM item.)
388
389           LATERAL can also precede a function-call FROM item, but in this
390           case it is a noise word, because the function expression can refer
391           to earlier FROM items in any case.
392
393           A LATERAL item can appear at top level in the FROM list, or within
394           a JOIN tree. In the latter case it can also refer to any items that
395           are on the left-hand side of a JOIN that it is on the right-hand
396           side of.
397
398           When a FROM item contains LATERAL cross-references, evaluation
399           proceeds as follows: for each row of the FROM item providing the
400           cross-referenced column(s), or set of rows of multiple FROM items
401           providing the columns, the LATERAL item is evaluated using that row
402           or row set's values of the columns. The resulting row(s) are joined
403           as usual with the rows they were computed from. This is repeated
404           for each row or set of rows from the column source table(s).
405
406           The column source table(s) must be INNER or LEFT joined to the
407           LATERAL item, else there would not be a well-defined set of rows
408           from which to compute each set of rows for the LATERAL item. Thus,
409           although a construct such as X RIGHT JOIN LATERAL Y is
410           syntactically valid, it is not actually allowed for Y to reference
411           X.
412
413   WHERE Clause
414       The optional WHERE clause has the general form
415
416           WHERE condition
417
418       where condition is any expression that evaluates to a result of type
419       boolean. Any row that does not satisfy this condition will be
420       eliminated from the output. A row satisfies the condition if it returns
421       true when the actual row values are substituted for any variable
422       references.
423
424   GROUP BY Clause
425       The optional GROUP BY clause has the general form
426
427           GROUP BY grouping_element [, ...]
428
429       GROUP BY will condense into a single row all selected rows that share
430       the same values for the grouped expressions. An expression used inside
431       a grouping_element can be an input column name, or the name or ordinal
432       number of an output column (SELECT list item), or an arbitrary
433       expression formed from input-column values. In case of ambiguity, a
434       GROUP BY name will be interpreted as an input-column name rather than
435       an output column name.
436
437       If any of GROUPING SETS, ROLLUP or CUBE are present as grouping
438       elements, then the GROUP BY clause as a whole defines some number of
439       independent grouping sets. The effect of this is equivalent to
440       constructing a UNION ALL between subqueries with the individual
441       grouping sets as their GROUP BY clauses. For further details on the
442       handling of grouping sets see Section 7.2.4.
443
444       Aggregate functions, if any are used, are computed across all rows
445       making up each group, producing a separate value for each group. (If
446       there are aggregate functions but no GROUP BY clause, the query is
447       treated as having a single group comprising all the selected rows.) The
448       set of rows fed to each aggregate function can be further filtered by
449       attaching a FILTER clause to the aggregate function call; see
450       Section 4.2.7 for more information. When a FILTER clause is present,
451       only those rows matching it are included in the input to that aggregate
452       function.
453
454       When GROUP BY is present, or any aggregate functions are present, it is
455       not valid for the SELECT list expressions to refer to ungrouped columns
456       except within aggregate functions or when the ungrouped column is
457       functionally dependent on the grouped columns, since there would
458       otherwise be more than one possible value to return for an ungrouped
459       column. A functional dependency exists if the grouped columns (or a
460       subset thereof) are the primary key of the table containing the
461       ungrouped column.
462
463       Keep in mind that all aggregate functions are evaluated before
464       evaluating any “scalar” expressions in the HAVING clause or SELECT
465       list. This means that, for example, a CASE expression cannot be used to
466       skip evaluation of an aggregate function; see Section 4.2.14.
467
468       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
469       cannot be specified with GROUP BY.
470
471   HAVING Clause
472       The optional HAVING clause has the general form
473
474           HAVING condition
475
476       where condition is the same as specified for the WHERE clause.
477
478       HAVING eliminates group rows that do not satisfy the condition.  HAVING
479       is different from WHERE: WHERE filters individual rows before the
480       application of GROUP BY, while HAVING filters group rows created by
481       GROUP BY. Each column referenced in condition must unambiguously
482       reference a grouping column, unless the reference appears within an
483       aggregate function or the ungrouped column is functionally dependent on
484       the grouping columns.
485
486       The presence of HAVING turns a query into a grouped query even if there
487       is no GROUP BY clause. This is the same as what happens when the query
488       contains aggregate functions but no GROUP BY clause. All the selected
489       rows are considered to form a single group, and the SELECT list and
490       HAVING clause can only reference table columns from within aggregate
491       functions. Such a query will emit a single row if the HAVING condition
492       is true, zero rows if it is not true.
493
494       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
495       cannot be specified with HAVING.
496
497   WINDOW Clause
498       The optional WINDOW clause has the general form
499
500           WINDOW window_name AS ( window_definition ) [, ...]
501
502       where window_name is a name that can be referenced from OVER clauses or
503       subsequent window definitions, and window_definition is
504
505           [ existing_window_name ]
506           [ PARTITION BY expression [, ...] ]
507           [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
508           [ frame_clause ]
509
510       If an existing_window_name is specified it must refer to an earlier
511       entry in the WINDOW list; the new window copies its partitioning clause
512       from that entry, as well as its ordering clause if any. In this case
513       the new window cannot specify its own PARTITION BY clause, and it can
514       specify ORDER BY only if the copied window does not have one. The new
515       window always uses its own frame clause; the copied window must not
516       specify a frame clause.
517
518       The elements of the PARTITION BY list are interpreted in much the same
519       fashion as elements of a GROUP BY Clause, except that they are always
520       simple expressions and never the name or number of an output column.
521       Another difference is that these expressions can contain aggregate
522       function calls, which are not allowed in a regular GROUP BY clause.
523       They are allowed here because windowing occurs after grouping and
524       aggregation.
525
526       Similarly, the elements of the ORDER BY list are interpreted in much
527       the same fashion as elements of an ORDER BY Clause, except that the
528       expressions are always taken as simple expressions and never the name
529       or number of an output column.
530
531       The optional frame_clause defines the window frame for window functions
532       that depend on the frame (not all do). The window frame is a set of
533       related rows for each row of the query (called the current row). The
534       frame_clause can be one of
535
536           { RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
537           { RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]
538
539       where frame_start and frame_end can be one of
540
541           UNBOUNDED PRECEDING
542           offset PRECEDING
543           CURRENT ROW
544           offset FOLLOWING
545           UNBOUNDED FOLLOWING
546
547       and frame_exclusion can be one of
548
549           EXCLUDE CURRENT ROW
550           EXCLUDE GROUP
551           EXCLUDE TIES
552           EXCLUDE NO OTHERS
553
554       If frame_end is omitted it defaults to CURRENT ROW. Restrictions are
555       that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be
556       UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in
557       the above list of frame_start and frame_end options than the
558       frame_start choice does — for example RANGE BETWEEN CURRENT ROW AND
559       offset PRECEDING is not allowed.
560
561       The default framing option is RANGE UNBOUNDED PRECEDING, which is the
562       same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the
563       frame to be all rows from the partition start up through the current
564       row's last peer (a row that the window's ORDER BY clause considers
565       equivalent to the current row; all rows are peers if there is no ORDER
566       BY). In general, UNBOUNDED PRECEDING means that the frame starts with
567       the first row of the partition, and similarly UNBOUNDED FOLLOWING means
568       that the frame ends with the last row of the partition, regardless of
569       RANGE, ROWS or GROUPS mode. In ROWS mode, CURRENT ROW means that the
570       frame starts or ends with the current row; but in RANGE or GROUPS mode
571       it means that the frame starts or ends with the current row's first or
572       last peer in the ORDER BY ordering. The offset PRECEDING and offset
573       FOLLOWING options vary in meaning depending on the frame mode. In ROWS
574       mode, the offset is an integer indicating that the frame starts or ends
575       that many rows before or after the current row. In GROUPS mode, the
576       offset is an integer indicating that the frame starts or ends that many
577       peer groups before or after the current row's peer group, where a peer
578       group is a group of rows that are equivalent according to the window's
579       ORDER BY clause. In RANGE mode, use of an offset option requires that
580       there be exactly one ORDER BY column in the window definition. Then the
581       frame contains those rows whose ordering column value is no more than
582       offset less than (for PRECEDING) or more than (for FOLLOWING) the
583       current row's ordering column value. In these cases the data type of
584       the offset expression depends on the data type of the ordering column.
585       For numeric ordering columns it is typically of the same type as the
586       ordering column, but for datetime ordering columns it is an interval.
587       In all these cases, the value of the offset must be non-null and
588       non-negative. Also, while the offset does not have to be a simple
589       constant, it cannot contain variables, aggregate functions, or window
590       functions.
591
592       The frame_exclusion option allows rows around the current row to be
593       excluded from the frame, even if they would be included according to
594       the frame start and frame end options.  EXCLUDE CURRENT ROW excludes
595       the current row from the frame.  EXCLUDE GROUP excludes the current row
596       and its ordering peers from the frame.  EXCLUDE TIES excludes any peers
597       of the current row from the frame, but not the current row itself.
598       EXCLUDE NO OTHERS simply specifies explicitly the default behavior of
599       not excluding the current row or its peers.
600
601       Beware that the ROWS mode can produce unpredictable results if the
602       ORDER BY ordering does not order the rows uniquely. The RANGE and
603       GROUPS modes are designed to ensure that rows that are peers in the
604       ORDER BY ordering are treated alike: all rows of a given peer group
605       will be in the frame or excluded from it.
606
607       The purpose of a WINDOW clause is to specify the behavior of window
608       functions appearing in the query's SELECT List or ORDER BY Clause.
609       These functions can reference the WINDOW clause entries by name in
610       their OVER clauses. A WINDOW clause entry does not have to be
611       referenced anywhere, however; if it is not used in the query it is
612       simply ignored. It is possible to use window functions without any
613       WINDOW clause at all, since a window function call can specify its
614       window definition directly in its OVER clause. However, the WINDOW
615       clause saves typing when the same window definition is needed for more
616       than one window function.
617
618       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
619       cannot be specified with WINDOW.
620
621       Window functions are described in detail in Section 3.5, Section 4.2.8,
622       and Section 7.2.5.
623
624   SELECT List
625       The SELECT list (between the key words SELECT and FROM) specifies
626       expressions that form the output rows of the SELECT statement. The
627       expressions can (and usually do) refer to columns computed in the FROM
628       clause.
629
630       Just as in a table, every output column of a SELECT has a name. In a
631       simple SELECT this name is just used to label the column for display,
632       but when the SELECT is a sub-query of a larger query, the name is seen
633       by the larger query as the column name of the virtual table produced by
634       the sub-query. To specify the name to use for an output column, write
635       AS output_name after the column's expression. (You can omit AS, but
636       only if the desired output name does not match any PostgreSQL keyword
637       (see Appendix C). For protection against possible future keyword
638       additions, it is recommended that you always either write AS or
639       double-quote the output name.) If you do not specify a column name, a
640       name is chosen automatically by PostgreSQL. If the column's expression
641       is a simple column reference then the chosen name is the same as that
642       column's name. In more complex cases a function or type name may be
643       used, or the system may fall back on a generated name such as ?column?.
644
645       An output column's name can be used to refer to the column's value in
646       ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses;
647       there you must write out the expression instead.
648
649       Instead of an expression, * can be written in the output list as a
650       shorthand for all the columns of the selected rows. Also, you can write
651       table_name.*  as a shorthand for the columns coming from just that
652       table. In these cases it is not possible to specify new names with AS;
653       the output column names will be the same as the table columns' names.
654
655       According to the SQL standard, the expressions in the output list
656       should be computed before applying DISTINCT, ORDER BY, or LIMIT. This
657       is obviously necessary when using DISTINCT, since otherwise it's not
658       clear what values are being made distinct. However, in many cases it is
659       convenient if output expressions are computed after ORDER BY and LIMIT;
660       particularly if the output list contains any volatile or expensive
661       functions. With that behavior, the order of function evaluations is
662       more intuitive and there will not be evaluations corresponding to rows
663       that never appear in the output.  PostgreSQL will effectively evaluate
664       output expressions after sorting and limiting, so long as those
665       expressions are not referenced in DISTINCT, ORDER BY or GROUP BY. (As a
666       counterexample, SELECT f(x) FROM tab ORDER BY 1 clearly must evaluate
667       f(x) before sorting.) Output expressions that contain set-returning
668       functions are effectively evaluated after sorting and before limiting,
669       so that LIMIT will act to cut off the output from a set-returning
670       function.
671
672           Note
673           PostgreSQL versions before 9.6 did not provide any guarantees about
674           the timing of evaluation of output expressions versus sorting and
675           limiting; it depended on the form of the chosen query plan.
676
677   DISTINCT Clause
678       If SELECT DISTINCT is specified, all duplicate rows are removed from
679       the result set (one row is kept from each group of duplicates).  SELECT
680       ALL specifies the opposite: all rows are kept; that is the default.
681
682       SELECT DISTINCT ON ( expression [, ...] ) keeps only the first row of
683       each set of rows where the given expressions evaluate to equal. The
684       DISTINCT ON expressions are interpreted using the same rules as for
685       ORDER BY (see above). Note that the “first row” of each set is
686       unpredictable unless ORDER BY is used to ensure that the desired row
687       appears first. For example:
688
689           SELECT DISTINCT ON (location) location, time, report
690               FROM weather_reports
691               ORDER BY location, time DESC;
692
693       retrieves the most recent weather report for each location. But if we
694       had not used ORDER BY to force descending order of time values for each
695       location, we'd have gotten a report from an unpredictable time for each
696       location.
697
698       The DISTINCT ON expression(s) must match the leftmost ORDER BY
699       expression(s). The ORDER BY clause will normally contain additional
700       expression(s) that determine the desired precedence of rows within each
701       DISTINCT ON group.
702
703       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
704       cannot be specified with DISTINCT.
705
706   UNION Clause
707       The UNION clause has this general form:
708
709           select_statement UNION [ ALL | DISTINCT ] select_statement
710
711       select_statement is any SELECT statement without an ORDER BY, LIMIT,
712       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
713       (ORDER BY and LIMIT can be attached to a subexpression if it is
714       enclosed in parentheses. Without parentheses, these clauses will be
715       taken to apply to the result of the UNION, not to its right-hand input
716       expression.)
717
718       The UNION operator computes the set union of the rows returned by the
719       involved SELECT statements. A row is in the set union of two result
720       sets if it appears in at least one of the result sets. The two SELECT
721       statements that represent the direct operands of the UNION must produce
722       the same number of columns, and corresponding columns must be of
723       compatible data types.
724
725       The result of UNION does not contain any duplicate rows unless the ALL
726       option is specified.  ALL prevents elimination of duplicates.
727       (Therefore, UNION ALL is usually significantly quicker than UNION; use
728       ALL when you can.)  DISTINCT can be written to explicitly specify the
729       default behavior of eliminating duplicate rows.
730
731       Multiple UNION operators in the same SELECT statement are evaluated
732       left to right, unless otherwise indicated by parentheses.
733
734       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
735       cannot be specified either for a UNION result or for any input of a
736       UNION.
737
738   INTERSECT Clause
739       The INTERSECT clause has this general form:
740
741           select_statement INTERSECT [ ALL | DISTINCT ] select_statement
742
743       select_statement is any SELECT statement without an ORDER BY, LIMIT,
744       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
745
746       The INTERSECT operator computes the set intersection of the rows
747       returned by the involved SELECT statements. A row is in the
748       intersection of two result sets if it appears in both result sets.
749
750       The result of INTERSECT does not contain any duplicate rows unless the
751       ALL option is specified. With ALL, a row that has m duplicates in the
752       left table and n duplicates in the right table will appear min(m,n)
753       times in the result set.  DISTINCT can be written to explicitly specify
754       the default behavior of eliminating duplicate rows.
755
756       Multiple INTERSECT operators in the same SELECT statement are evaluated
757       left to right, unless parentheses dictate otherwise.  INTERSECT binds
758       more tightly than UNION. That is, A UNION B INTERSECT C will be read as
759       A UNION (B INTERSECT C).
760
761       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
762       cannot be specified either for an INTERSECT result or for any input of
763       an INTERSECT.
764
765   EXCEPT Clause
766       The EXCEPT clause has this general form:
767
768           select_statement EXCEPT [ ALL | DISTINCT ] select_statement
769
770       select_statement is any SELECT statement without an ORDER BY, LIMIT,
771       FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, or FOR KEY SHARE clause.
772
773       The EXCEPT operator computes the set of rows that are in the result of
774       the left SELECT statement but not in the result of the right one.
775
776       The result of EXCEPT does not contain any duplicate rows unless the ALL
777       option is specified. With ALL, a row that has m duplicates in the left
778       table and n duplicates in the right table will appear max(m-n,0) times
779       in the result set.  DISTINCT can be written to explicitly specify the
780       default behavior of eliminating duplicate rows.
781
782       Multiple EXCEPT operators in the same SELECT statement are evaluated
783       left to right, unless parentheses dictate otherwise.  EXCEPT binds at
784       the same level as UNION.
785
786       Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and FOR KEY SHARE
787       cannot be specified either for an EXCEPT result or for any input of an
788       EXCEPT.
789
790   ORDER BY Clause
791       The optional ORDER BY clause has this general form:
792
793           ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]
794
795       The ORDER BY clause causes the result rows to be sorted according to
796       the specified expression(s). If two rows are equal according to the
797       leftmost expression, they are compared according to the next expression
798       and so on. If they are equal according to all specified expressions,
799       they are returned in an implementation-dependent order.
800
801       Each expression can be the name or ordinal number of an output column
802       (SELECT list item), or it can be an arbitrary expression formed from
803       input-column values.
804
805       The ordinal number refers to the ordinal (left-to-right) position of
806       the output column. This feature makes it possible to define an ordering
807       on the basis of a column that does not have a unique name. This is
808       never absolutely necessary because it is always possible to assign a
809       name to an output column using the AS clause.
810
811       It is also possible to use arbitrary expressions in the ORDER BY
812       clause, including columns that do not appear in the SELECT output list.
813       Thus the following statement is valid:
814
815           SELECT name FROM distributors ORDER BY code;
816
817       A limitation of this feature is that an ORDER BY clause applying to the
818       result of a UNION, INTERSECT, or EXCEPT clause can only specify an
819       output column name or number, not an expression.
820
821       If an ORDER BY expression is a simple name that matches both an output
822       column name and an input column name, ORDER BY will interpret it as the
823       output column name. This is the opposite of the choice that GROUP BY
824       will make in the same situation. This inconsistency is made to be
825       compatible with the SQL standard.
826
827       Optionally one can add the key word ASC (ascending) or DESC
828       (descending) after any expression in the ORDER BY clause. If not
829       specified, ASC is assumed by default. Alternatively, a specific
830       ordering operator name can be specified in the USING clause. An
831       ordering operator must be a less-than or greater-than member of some
832       B-tree operator family.  ASC is usually equivalent to USING < and DESC
833       is usually equivalent to USING >. (But the creator of a user-defined
834       data type can define exactly what the default sort ordering is, and it
835       might correspond to operators with other names.)
836
837       If NULLS LAST is specified, null values sort after all non-null values;
838       if NULLS FIRST is specified, null values sort before all non-null
839       values. If neither is specified, the default behavior is NULLS LAST
840       when ASC is specified or implied, and NULLS FIRST when DESC is
841       specified (thus, the default is to act as though nulls are larger than
842       non-nulls). When USING is specified, the default nulls ordering depends
843       on whether the operator is a less-than or greater-than operator.
844
845       Note that ordering options apply only to the expression they follow;
846       for example ORDER BY x, y DESC does not mean the same thing as ORDER BY
847       x DESC, y DESC.
848
849       Character-string data is sorted according to the collation that applies
850       to the column being sorted. That can be overridden at need by including
851       a COLLATE clause in the expression, for example ORDER BY mycolumn
852       COLLATE "en_US". For more information see Section 4.2.10 and
853       Section 23.2.
854
855   LIMIT Clause
856       The LIMIT clause consists of two independent sub-clauses:
857
858           LIMIT { count | ALL }
859           OFFSET start
860
861
862       count specifies the maximum number of rows to return, while start
863       specifies the number of rows to skip before starting to return rows.
864       When both are specified, start rows are skipped before starting to
865       count the count rows to be returned.
866
867       If the count expression evaluates to NULL, it is treated as LIMIT ALL,
868       i.e., no limit. If start evaluates to NULL, it is treated the same as
869       OFFSET 0.
870
871       SQL:2008 introduced a different syntax to achieve the same result,
872       which PostgreSQL also supports. It is:
873
874           OFFSET start { ROW | ROWS }
875           FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
876
877       In this syntax, the start or count value is required by the standard to
878       be a literal constant, a parameter, or a variable name; as a PostgreSQL
879       extension, other expressions are allowed, but will generally need to be
880       enclosed in parentheses to avoid ambiguity. If count is omitted in a
881       FETCH clause, it defaults to 1.  ROW and ROWS as well as FIRST and NEXT
882       are noise words that don't influence the effects of these clauses.
883       According to the standard, the OFFSET clause must come before the FETCH
884       clause if both are present; but PostgreSQL is laxer and allows either
885       order.
886
887       When using LIMIT, it is a good idea to use an ORDER BY clause that
888       constrains the result rows into a unique order. Otherwise you will get
889       an unpredictable subset of the query's rows — you might be asking for
890       the tenth through twentieth rows, but tenth through twentieth in what
891       ordering? You don't know what ordering unless you specify ORDER BY.
892
893       The query planner takes LIMIT into account when generating a query
894       plan, so you are very likely to get different plans (yielding different
895       row orders) depending on what you use for LIMIT and OFFSET. Thus, using
896       different LIMIT/OFFSET values to select different subsets of a query
897       result will give inconsistent results unless you enforce a predictable
898       result ordering with ORDER BY. This is not a bug; it is an inherent
899       consequence of the fact that SQL does not promise to deliver the
900       results of a query in any particular order unless ORDER BY is used to
901       constrain the order.
902
903       It is even possible for repeated executions of the same LIMIT query to
904       return different subsets of the rows of a table, if there is not an
905       ORDER BY to enforce selection of a deterministic subset. Again, this is
906       not a bug; determinism of the results is simply not guaranteed in such
907       a case.
908
909   The Locking Clause
910       FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE are locking
911       clauses; they affect how SELECT locks rows as they are obtained from
912       the table.
913
914       The locking clause has the general form
915
916           FOR lock_strength [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ]
917
918       where lock_strength can be one of
919
920           UPDATE
921           NO KEY UPDATE
922           SHARE
923           KEY SHARE
924
925       For more information on each row-level lock mode, refer to
926       Section 13.3.2.
927
928       To prevent the operation from waiting for other transactions to commit,
929       use either the NOWAIT or SKIP LOCKED option. With NOWAIT, the statement
930       reports an error, rather than waiting, if a selected row cannot be
931       locked immediately. With SKIP LOCKED, any selected rows that cannot be
932       immediately locked are skipped. Skipping locked rows provides an
933       inconsistent view of the data, so this is not suitable for general
934       purpose work, but can be used to avoid lock contention with multiple
935       consumers accessing a queue-like table. Note that NOWAIT and SKIP
936       LOCKED apply only to the row-level lock(s) — the required ROW SHARE
937       table-level lock is still taken in the ordinary way (see Chapter 13).
938       You can use LOCK(7) with the NOWAIT option first, if you need to
939       acquire the table-level lock without waiting.
940
941       If specific tables are named in a locking clause, then only rows coming
942       from those tables are locked; any other tables used in the SELECT are
943       simply read as usual. A locking clause without a table list affects all
944       tables used in the statement. If a locking clause is applied to a view
945       or sub-query, it affects all tables used in the view or sub-query.
946       However, these clauses do not apply to WITH queries referenced by the
947       primary query. If you want row locking to occur within a WITH query,
948       specify a locking clause within the WITH query.
949
950       Multiple locking clauses can be written if it is necessary to specify
951       different locking behavior for different tables. If the same table is
952       mentioned (or implicitly affected) by more than one locking clause,
953       then it is processed as if it was only specified by the strongest one.
954       Similarly, a table is processed as NOWAIT if that is specified in any
955       of the clauses affecting it. Otherwise, it is processed as SKIP LOCKED
956       if that is specified in any of the clauses affecting it.
957
958       The locking clauses cannot be used in contexts where returned rows
959       cannot be clearly identified with individual table rows; for example
960       they cannot be used with aggregation.
961
962       When a locking clause appears at the top level of a SELECT query, the
963       rows that are locked are exactly those that are returned by the query;
964       in the case of a join query, the rows locked are those that contribute
965       to returned join rows. In addition, rows that satisfied the query
966       conditions as of the query snapshot will be locked, although they will
967       not be returned if they were updated after the snapshot and no longer
968       satisfy the query conditions. If a LIMIT is used, locking stops once
969       enough rows have been returned to satisfy the limit (but note that rows
970       skipped over by OFFSET will get locked). Similarly, if a locking clause
971       is used in a cursor's query, only rows actually fetched or stepped past
972       by the cursor will be locked.
973
974       When a locking clause appears in a sub-SELECT, the rows locked are
975       those returned to the outer query by the sub-query. This might involve
976       fewer rows than inspection of the sub-query alone would suggest, since
977       conditions from the outer query might be used to optimize execution of
978       the sub-query. For example,
979
980           SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
981
982       will lock only rows having col1 = 5, even though that condition is not
983       textually within the sub-query.
984
985       Previous releases failed to preserve a lock which is upgraded by a
986       later savepoint. For example, this code:
987
988           BEGIN;
989           SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
990           SAVEPOINT s;
991           UPDATE mytable SET ... WHERE key = 1;
992           ROLLBACK TO s;
993
994       would fail to preserve the FOR UPDATE lock after the ROLLBACK TO. This
995       has been fixed in release 9.3.
996
997           Caution
998           It is possible for a SELECT command running at the READ COMMITTED
999           transaction isolation level and using ORDER BY and a locking clause
1000           to return rows out of order. This is because ORDER BY is applied
1001           first. The command sorts the result, but might then block trying to
1002           obtain a lock on one or more of the rows. Once the SELECT unblocks,
1003           some of the ordering column values might have been modified,
1004           leading to those rows appearing to be out of order (though they are
1005           in order in terms of the original column values). This can be
1006           worked around at need by placing the FOR UPDATE/SHARE clause in a
1007           sub-query, for example
1008
1009               SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
1010
1011           Note that this will result in locking all rows of mytable, whereas
1012           FOR UPDATE at the top level would lock only the actually returned
1013           rows. This can make for a significant performance difference,
1014           particularly if the ORDER BY is combined with LIMIT or other
1015           restrictions. So this technique is recommended only if concurrent
1016           updates of the ordering columns are expected and a strictly sorted
1017           result is required.
1018
1019           At the REPEATABLE READ or SERIALIZABLE transaction isolation level
1020           this would cause a serialization failure (with a SQLSTATE of
1021           '40001'), so there is no possibility of receiving rows out of order
1022           under these isolation levels.
1023
1024   TABLE Command
1025       The command
1026
1027           TABLE name
1028
1029       is equivalent to
1030
1031           SELECT * FROM name
1032
1033       It can be used as a top-level command or as a space-saving syntax
1034       variant in parts of complex queries. Only the WITH, UNION, INTERSECT,
1035       EXCEPT, ORDER BY, LIMIT, OFFSET, FETCH and FOR locking clauses can be
1036       used with TABLE; the WHERE clause and any form of aggregation cannot be
1037       used.
1038

EXAMPLES

1040       To join the table films with the table distributors:
1041
1042           SELECT f.title, f.did, d.name, f.date_prod, f.kind
1043               FROM distributors d, films f
1044               WHERE f.did = d.did
1045
1046                  title       | did |     name     | date_prod  |   kind
1047           -------------------+-----+--------------+------------+----------
1048            The Third Man     | 101 | British Lion | 1949-12-23 | Drama
1049            The African Queen | 101 | British Lion | 1951-08-11 | Romantic
1050            ...
1051
1052       To sum the column len of all films and group the results by kind:
1053
1054           SELECT kind, sum(len) AS total FROM films GROUP BY kind;
1055
1056              kind   | total
1057           ----------+-------
1058            Action   | 07:34
1059            Comedy   | 02:58
1060            Drama    | 14:28
1061            Musical  | 06:42
1062            Romantic | 04:38
1063
1064       To sum the column len of all films, group the results by kind and show
1065       those group totals that are less than 5 hours:
1066
1067           SELECT kind, sum(len) AS total
1068               FROM films
1069               GROUP BY kind
1070               HAVING sum(len) < interval '5 hours';
1071
1072              kind   | total
1073           ----------+-------
1074            Comedy   | 02:58
1075            Romantic | 04:38
1076
1077       The following two examples are identical ways of sorting the individual
1078       results according to the contents of the second column (name):
1079
1080           SELECT * FROM distributors ORDER BY name;
1081           SELECT * FROM distributors ORDER BY 2;
1082
1083            did |       name
1084           -----+------------------
1085            109 | 20th Century Fox
1086            110 | Bavaria Atelier
1087            101 | British Lion
1088            107 | Columbia
1089            102 | Jean Luc Godard
1090            113 | Luso films
1091            104 | Mosfilm
1092            103 | Paramount
1093            106 | Toho
1094            105 | United Artists
1095            111 | Walt Disney
1096            112 | Warner Bros.
1097            108 | Westward
1098
1099       The next example shows how to obtain the union of the tables
1100       distributors and actors, restricting the results to those that begin
1101       with the letter W in each table. Only distinct rows are wanted, so the
1102       key word ALL is omitted.
1103
1104           distributors:               actors:
1105            did |     name              id |     name
1106           -----+--------------        ----+----------------
1107            108 | Westward               1 | Woody Allen
1108            111 | Walt Disney            2 | Warren Beatty
1109            112 | Warner Bros.           3 | Walter Matthau
1110            ...                         ...
1111
1112           SELECT distributors.name
1113               FROM distributors
1114               WHERE distributors.name LIKE 'W%'
1115           UNION
1116           SELECT actors.name
1117               FROM actors
1118               WHERE actors.name LIKE 'W%';
1119
1120                 name
1121           ----------------
1122            Walt Disney
1123            Walter Matthau
1124            Warner Bros.
1125            Warren Beatty
1126            Westward
1127            Woody Allen
1128
1129       This example shows how to use a function in the FROM clause, both with
1130       and without a column definition list:
1131
1132           CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
1133               SELECT * FROM distributors WHERE did = $1;
1134           $$ LANGUAGE SQL;
1135
1136           SELECT * FROM distributors(111);
1137            did |    name
1138           -----+-------------
1139            111 | Walt Disney
1140
1141           CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
1142               SELECT * FROM distributors WHERE did = $1;
1143           $$ LANGUAGE SQL;
1144
1145           SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
1146            f1  |     f2
1147           -----+-------------
1148            111 | Walt Disney
1149
1150       Here is an example of a function with an ordinality column added:
1151
1152           SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY;
1153            unnest | ordinality
1154           --------+----------
1155            a      |        1
1156            b      |        2
1157            c      |        3
1158            d      |        4
1159            e      |        5
1160            f      |        6
1161           (6 rows)
1162
1163       This example shows how to use a simple WITH clause:
1164
1165           WITH t AS (
1166               SELECT random() as x FROM generate_series(1, 3)
1167             )
1168           SELECT * FROM t
1169           UNION ALL
1170           SELECT * FROM t
1171
1172                    x
1173           --------------------
1174             0.534150459803641
1175             0.520092216785997
1176            0.0735620250925422
1177             0.534150459803641
1178             0.520092216785997
1179            0.0735620250925422
1180
1181       Notice that the WITH query was evaluated only once, so that we got two
1182       sets of the same three random values.
1183
1184       This example uses WITH RECURSIVE to find all subordinates (direct or
1185       indirect) of the employee Mary, and their level of indirectness, from a
1186       table that shows only direct subordinates:
1187
1188           WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
1189               SELECT 1, employee_name, manager_name
1190               FROM employee
1191               WHERE manager_name = 'Mary'
1192             UNION ALL
1193               SELECT er.distance + 1, e.employee_name, e.manager_name
1194               FROM employee_recursive er, employee e
1195               WHERE er.employee_name = e.manager_name
1196             )
1197           SELECT distance, employee_name FROM employee_recursive;
1198
1199       Notice the typical form of recursive queries: an initial condition,
1200       followed by UNION, followed by the recursive part of the query. Be sure
1201       that the recursive part of the query will eventually return no tuples,
1202       or else the query will loop indefinitely. (See Section 7.8 for more
1203       examples.)
1204
1205       This example uses LATERAL to apply a set-returning function
1206       get_product_names() for each row of the manufacturers table:
1207
1208           SELECT m.name AS mname, pname
1209           FROM manufacturers m, LATERAL get_product_names(m.id) pname;
1210
1211       Manufacturers not currently having any products would not appear in the
1212       result, since it is an inner join. If we wished to include the names of
1213       such manufacturers in the result, we could do:
1214
1215           SELECT m.name AS mname, pname
1216           FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;
1217

COMPATIBILITY

1219       Of course, the SELECT statement is compatible with the SQL standard.
1220       But there are some extensions and some missing features.
1221
1222   Omitted FROM Clauses
1223       PostgreSQL allows one to omit the FROM clause. It has a straightforward
1224       use to compute the results of simple expressions:
1225
1226           SELECT 2+2;
1227
1228            ?column?
1229           ----------
1230                   4
1231
1232       Some other SQL databases cannot do this except by introducing a dummy
1233       one-row table from which to do the SELECT.
1234
1235       Note that if a FROM clause is not specified, the query cannot reference
1236       any database tables. For example, the following query is invalid:
1237
1238           SELECT distributors.* WHERE distributors.name = 'Westward';
1239
1240
1241       PostgreSQL releases prior to 8.1 would accept queries of this form, and
1242       add an implicit entry to the query's FROM clause for each table
1243       referenced by the query. This is no longer allowed.
1244
1245   Empty SELECT Lists
1246       The list of output expressions after SELECT can be empty, producing a
1247       zero-column result table. This is not valid syntax according to the SQL
1248       standard.  PostgreSQL allows it to be consistent with allowing
1249       zero-column tables. However, an empty list is not allowed when DISTINCT
1250       is used.
1251
1252   Omitting the AS Key Word
1253       In the SQL standard, the optional key word AS can be omitted before an
1254       output column name whenever the new column name is a valid column name
1255       (that is, not the same as any reserved keyword).  PostgreSQL is
1256       slightly more restrictive: AS is required if the new column name
1257       matches any keyword at all, reserved or not. Recommended practice is to
1258       use AS or double-quote output column names, to prevent any possible
1259       conflict against future keyword additions.
1260
1261       In FROM items, both the standard and PostgreSQL allow AS to be omitted
1262       before an alias that is an unreserved keyword. But this is impractical
1263       for output column names, because of syntactic ambiguities.
1264
1265   ONLY and Inheritance
1266       The SQL standard requires parentheses around the table name when
1267       writing ONLY, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE
1268       ....  PostgreSQL considers these parentheses to be optional.
1269
1270       PostgreSQL allows a trailing * to be written to explicitly specify the
1271       non-ONLY behavior of including child tables. The standard does not
1272       allow this.
1273
1274       (These points apply equally to all SQL commands supporting the ONLY
1275       option.)
1276
1277   TABLESAMPLE Clause Restrictions
1278       The TABLESAMPLE clause is currently accepted only on regular tables and
1279       materialized views. According to the SQL standard it should be possible
1280       to apply it to any FROM item.
1281
1282   Function Calls in FROM
1283       PostgreSQL allows a function call to be written directly as a member of
1284       the FROM list. In the SQL standard it would be necessary to wrap such a
1285       function call in a sub-SELECT; that is, the syntax FROM func(...) alias
1286       is approximately equivalent to FROM LATERAL (SELECT func(...)) alias.
1287       Note that LATERAL is considered to be implicit; this is because the
1288       standard requires LATERAL semantics for an UNNEST() item in FROM.
1289       PostgreSQL treats UNNEST() the same as other set-returning functions.
1290
1291   Namespace Available to GROUP BY and ORDER BY
1292       In the SQL-92 standard, an ORDER BY clause can only use output column
1293       names or numbers, while a GROUP BY clause can only use expressions
1294       based on input column names.  PostgreSQL extends each of these clauses
1295       to allow the other choice as well (but it uses the standard's
1296       interpretation if there is ambiguity).  PostgreSQL also allows both
1297       clauses to specify arbitrary expressions. Note that names appearing in
1298       an expression will always be taken as input-column names, not as
1299       output-column names.
1300
1301       SQL:1999 and later use a slightly different definition which is not
1302       entirely upward compatible with SQL-92. In most cases, however,
1303       PostgreSQL will interpret an ORDER BY or GROUP BY expression the same
1304       way SQL:1999 does.
1305
1306   Functional Dependencies
1307       PostgreSQL recognizes functional dependency (allowing columns to be
1308       omitted from GROUP BY) only when a table's primary key is included in
1309       the GROUP BY list. The SQL standard specifies additional conditions
1310       that should be recognized.
1311
1312   LIMIT and OFFSET
1313       The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used
1314       by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ...
1315       FETCH {FIRST|NEXT} ...  for the same functionality, as shown above in
1316       LIMIT Clause. This syntax is also used by IBM DB2. (Applications
1317       written for Oracle frequently use a workaround involving the
1318       automatically generated rownum column, which is not available in
1319       PostgreSQL, to implement the effects of these clauses.)
1320
1321   FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, FOR KEY SHARE
1322       Although FOR UPDATE appears in the SQL standard, the standard allows it
1323       only as an option of DECLARE CURSOR.  PostgreSQL allows it in any
1324       SELECT query as well as in sub-SELECTs, but this is an extension. The
1325       FOR NO KEY UPDATE, FOR SHARE and FOR KEY SHARE variants, as well as the
1326       NOWAIT and SKIP LOCKED options, do not appear in the standard.
1327
1328   Data-Modifying Statements in WITH
1329       PostgreSQL allows INSERT, UPDATE, and DELETE to be used as WITH
1330       queries. This is not found in the SQL standard.
1331
1332   Nonstandard Clauses
1333       DISTINCT ON ( ... ) is an extension of the SQL standard.
1334
1335       ROWS FROM( ... ) is an extension of the SQL standard.
1336
1337       The MATERIALIZED and NOT MATERIALIZED options of WITH are extensions of
1338       the SQL standard.
1339
1340
1341
1342PostgreSQL 12.6                      2021                            SELECT(7)
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