1SELECT(7) SQL Commands SELECT(7)
2
6 SELECT, TABLE, WITH - retrieve rows from a table or view
7
10 [ WITH [ RECURSIVE ] with_query [, ...] ]
11 SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
12 * | expression [ [ AS ] output_name ] [, ...]
13 [ FROM from_item [, ...] ]
14 [ WHERE condition ]
15 [ GROUP BY expression [, ...] ]
16 [ HAVING condition [, ...] ]
17 [ WINDOW window_name AS ( window_definition ) [, ...] ]
18 [ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]
19 [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
20 [ LIMIT { count | ALL } ]
21 [ OFFSET start [ ROW | ROWS ] ]
22 [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
23 [ FOR { UPDATE | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] [...] ]
24 where from_item can be one of:
26 [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
28 ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
29 with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
30 function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
31 function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
32 from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]
33 and with_query is:
35 with_query_name [ ( column_name [, ...] ) ] AS ( select )
37 TABLE { [ ONLY ] table_name [ * ] | with_query_name }
39
42 SELECT retrieves rows from zero or more tables. The general processing
43 of SELECT is as follows:
44 1. All queries in the WITH list are computed. These effectively
46 serve as temporary tables that can be referenced in the FROM
47 list. A WITH query that is referenced more than once in FROM is
48 computed only once. (See WITH Clause [select(7)] below.)
49 2. All elements in the FROM list are computed. (Each element in
51 the FROM list is a real or virtual table.) If more than one ele‐
52 ment is specified in the FROM list, they are cross-joined
53 together. (See FROM Clause [select(7)] below.)
54 3. If the WHERE clause is specified, all rows that do not satisfy
56 the condition are eliminated from the output. (See WHERE Clause
57 [select(7)] below.)
58 4. If the GROUP BY clause is specified, the output is divided into
60 groups of rows that match on one or more values. If the HAVING
61 clause is present, it eliminates groups that do not satisfy the
62 given condition. (See GROUP BY Clause [select(7)] and HAVING
63 Clause [select(7)] below.)
64 5. The actual output rows are computed using the SELECT output
66 expressions for each selected row. (See SELECT List [select(7)]
67 below.)
68 6. Using the operators UNION, INTERSECT, and EXCEPT, the output of
70 more than one SELECT statement can be combined to form a single
71 result set. The UNION operator returns all rows that are in one
72 or both of the result sets. The INTERSECT operator returns all
73 rows that are strictly in both result sets. The EXCEPT operator
74 returns the rows that are in the first result set but not in the
75 second. In all three cases, duplicate rows are eliminated unless
76 ALL is specified. (See UNION Clause [select(7)], INTERSECT
77 Clause [select(7)], and EXCEPT Clause [select(7)] below.)
78 7. If the ORDER BY clause is specified, the returned rows are
80 sorted in the specified order. If ORDER BY is not given, the
81 rows are returned in whatever order the system finds fastest to
82 produce. (See ORDER BY Clause [select(7)] below.)
83 8. DISTINCT eliminates duplicate rows from the result. DISTINCT ON
85 eliminates rows that match on all the specified expressions. ALL
86 (the default) will return all candidate rows, including dupli‐
87 cates. (See DISTINCT Clause [select(7)] below.)
88 9. If the LIMIT (or FETCH FIRST) or OFFSET clause is specified, the
90 SELECT statement only returns a subset of the result rows. (See
91 LIMIT Clause [select(7)] below.)
92 10. If FOR UPDATE or FOR SHARE is specified, the SELECT statement
94 locks the selected rows against concurrent updates. (See FOR
95 UPDATE/FOR SHARE Clause [select(7)] below.)
96 You must have SELECT privilege on each column used in a SELECT command.
98 The use of FOR UPDATE or FOR SHARE requires UPDATE privilege as well
99 (for at least one column of each table so selected).
100
102 WITH CLAUSE
103 The WITH clause allows you to specify one or more subqueries that can
104 be referenced by name in the primary query. The subqueries effectively
105 act as temporary tables or views for the duration of the primary query.
106 A name (without schema qualification) must be specified for each WITH
108 query. Optionally, a list of column names can be specified; if this is
109 omitted, the column names are inferred from the subquery.
110 If RECURSIVE is specified, it allows a subquery to reference itself by
112 name. Such a subquery must have the form
113 non_recursive_term UNION [ ALL ] recursive_term
115 where the recursive self-reference must appear on the right-hand side
117 of the UNION. Only one recursive self-reference is permitted per query.
118 Another effect of RECURSIVE is that WITH queries need not be ordered: a
120 query can reference another one that is later in the list. (However,
121 circular references, or mutual recursion, are not implemented.) With‐
122 out RECURSIVE, WITH queries can only reference sibling WITH queries
123 that are earlier in the WITH list.
124 A useful property of WITH queries is that they are evaluated only once
126 per execution of the primary query, even if the primary query refers to
127 them more than once.
128 See in the documentation for additional information.
130 FROM CLAUSE
132 The FROM clause specifies one or more source tables for the SELECT. If
133 multiple sources are specified, the result is the Cartesian product
134 (cross join) of all the sources. But usually qualification conditions
135 are added to restrict the returned rows to a small subset of the Carte‐
136 sian product.
137 The FROM clause can contain the following elements:
139 table_name
141 The name (optionally schema-qualified) of an existing table or
142 view. If ONLY is specified before the table name, only that ta‐
143 ble is scanned. If ONLY is not specified, the table and all its
144 descendant tables (if any) are scanned. Optionally, * can be
145 specified after the table name to explicitly indicate that
146 descendant tables are included.
147 alias A substitute name for the FROM item containing the alias. An
149 alias is used for brevity or to eliminate ambiguity for self-
150 joins (where the same table is scanned multiple times). When an
151 alias is provided, it completely hides the actual name of the
152 table or function; for example given FROM foo AS f, the remain‐
153 der of the SELECT must refer to this FROM item as f not foo. If
154 an alias is written, a column alias list can also be written to
155 provide substitute names for one or more columns of the table.
156 select A sub-SELECT can appear in the FROM clause. This acts as though
158 its output were created as a temporary table for the duration of
159 this single SELECT command. Note that the sub-SELECT must be
160 surrounded by parentheses, and an alias must be provided for it.
161 A VALUES [values(7)] command can also be used here.
162 with_query_name
164 A WITH query is referenced by writing its name, just as though
165 the query's name were a table name. (In fact, the WITH query
166 hides any real table of the same name for the purposes of the
167 primary query. If necessary, you can refer to a real table of
168 the same name by schema-qualifying the table's name.) An alias
169 can be provided in the same way as for a table.
170 function_name
172 Function calls can appear in the FROM clause. (This is espe‐
173 cially useful for functions that return result sets, but any
174 function can be used.) This acts as though its output were cre‐
175 ated as a temporary table for the duration of this single SELECT
176 command. An alias can also be used. If an alias is written, a
177 column alias list can also be written to provide substitute
178 names for one or more attributes of the function's composite
179 return type. If the function has been defined as returning the
180 record data type, then an alias or the key word AS must be
181 present, followed by a column definition list in the form ( col‐
182 umn_name data_type [, ... ] ). The column definition list must
183 match the actual number and types of columns returned by the
184 function.
185 join_type
187 One of
188 · [ INNER ] JOIN
190 · LEFT [ OUTER ] JOIN
192 · RIGHT [ OUTER ] JOIN
194 · FULL [ OUTER ] JOIN
196 · CROSS JOIN
198 For the INNER and OUTER join types, a join condition must be specified,
200 namely exactly one of NATURAL, ON join_condition, or USING (join_column
201 [, ...]). See below for the meaning. For CROSS JOIN, none of these
202 clauses can appear.
203 A JOIN clause combines two FROM items. Use parentheses if necessary to
205 determine the order of nesting. In the absence of parentheses, JOINs
206 nest left-to-right. In any case JOIN binds more tightly than the commas
207 separating FROM items.
208 CROSS JOIN and INNER JOIN produce a simple Cartesian product, the same
210 result as you get from listing the two items at the top level of FROM,
211 but restricted by the join condition (if any). CROSS JOIN is equiva‐
212 lent to INNER JOIN ON (TRUE), that is, no rows are removed by qualifi‐
213 cation. These join types are just a notational convenience, since they
214 do nothing you couldn't do with plain FROM and WHERE.
215 LEFT OUTER JOIN returns all rows in the qualified Cartesian product
217 (i.e., all combined rows that pass its join condition), plus one copy
218 of each row in the left-hand table for which there was no right-hand
219 row that passed the join condition. This left-hand row is extended to
220 the full width of the joined table by inserting null values for the
221 right-hand columns. Note that only the JOIN clause's own condition is
222 considered while deciding which rows have matches. Outer conditions are
223 applied afterwards.
224 Conversely, RIGHT OUTER JOIN returns all the joined rows, plus one row
226 for each unmatched right-hand row (extended with nulls on the left).
227 This is just a notational convenience, since you could convert it to a
228 LEFT OUTER JOIN by switching the left and right inputs.
229 FULL OUTER JOIN returns all the joined rows, plus one row for each
231 unmatched left-hand row (extended with nulls on the right), plus one
232 row for each unmatched right-hand row (extended with nulls on the
233 left).
234 ON join_condition
236 join_condition is an expression resulting in a value of type
237 boolean (similar to a WHERE clause) that specifies which rows in
238 a join are considered to match.
239 USING ( join_column [, ...] )
241 A clause of the form USING ( a, b, ... ) is shorthand for ON
242 left_table.a = right_table.a AND left_table.b = right_table.b
243 .... Also, USING implies that only one of each pair of equiva‐
244 lent columns will be included in the join output, not both.
245 NATURAL
247 NATURAL is shorthand for a USING list that mentions all columns
248 in the two tables that have the same names.
249 WHERE CLAUSE
251 The optional WHERE clause has the general form
252 WHERE condition
254 where condition is any expression that evaluates to a result of type
256 boolean. Any row that does not satisfy this condition will be elimi‐
257 nated from the output. A row satisfies the condition if it returns true
258 when the actual row values are substituted for any variable references.
259 GROUP BY CLAUSE
261 The optional GROUP BY clause has the general form
262 GROUP BY expression [, ...]
264 GROUP BY will condense into a single row all selected rows that share
267 the same values for the grouped expressions. expression can be an input
268 column name, or the name or ordinal number of an output column (SELECT
269 list item), or an arbitrary expression formed from input-column values.
270 In case of ambiguity, a GROUP BY name will be interpreted as an input-
271 column name rather than an output column name.
272 Aggregate functions, if any are used, are computed across all rows mak‐
274 ing up each group, producing a separate value for each group (whereas
275 without GROUP BY, an aggregate produces a single value computed across
276 all the selected rows). When GROUP BY is present, it is not valid for
277 the SELECT list expressions to refer to ungrouped columns except within
278 aggregate functions, since there would be more than one possible value
279 to return for an ungrouped column.
280 HAVING CLAUSE
282 The optional HAVING clause has the general form
283 HAVING condition
285 where condition is the same as specified for the WHERE clause.
287 HAVING eliminates group rows that do not satisfy the condition. HAVING
289 is different from WHERE: WHERE filters individual rows before the
290 application of GROUP BY, while HAVING filters group rows created by
291 GROUP BY. Each column referenced in condition must unambiguously refer‐
292 ence a grouping column, unless the reference appears within an aggre‐
293 gate function.
294 The presence of HAVING turns a query into a grouped query even if there
296 is no GROUP BY clause. This is the same as what happens when the query
297 contains aggregate functions but no GROUP BY clause. All the selected
298 rows are considered to form a single group, and the SELECT list and
299 HAVING clause can only reference table columns from within aggregate
300 functions. Such a query will emit a single row if the HAVING condition
301 is true, zero rows if it is not true.
302 WINDOW CLAUSE
304 The optional WINDOW clause has the general form
305 WINDOW window_name AS ( window_definition ) [, ...]
307 where window_name is a name that can be referenced from OVER clauses or
309 subsequent window definitions, and window_definition is
310 [ existing_window_name ]
312 [ PARTITION BY expression [, ...] ]
313 [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
314 [ frame_clause ]
315 If an existing_window_name is specified it must refer to an earlier
318 entry in the WINDOW list; the new window copies its partitioning clause
319 from that entry, as well as its ordering clause if any. In this case
320 the new window cannot specify its own PARTITION BY clause, and it can
321 specify ORDER BY only if the copied window does not have one. The new
322 window always uses its own frame clause; the copied window must not
323 specify a frame clause.
324 The elements of the PARTITION BY list are interpreted in much the same
326 fashion as elements of a GROUP BY Clause [select(7)], except that they
327 are always simple expressions and never the name or number of an output
328 column. Another difference is that these expressions can contain
329 aggregate function calls, which are not allowed in a regular GROUP BY
330 clause. They are allowed here because windowing occurs after grouping
331 and aggregation.
332 Similarly, the elements of the ORDER BY list are interpreted in much
334 the same fashion as elements of an ORDER BY Clause [select(7)], except
335 that the expressions are always taken as simple expressions and never
336 the name or number of an output column.
337 The optional frame_clause defines the window frame for window functions
339 that depend on the frame (not all do). It can be one of
340 RANGE UNBOUNDED PRECEDING
342 RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
343 RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
344 ROWS UNBOUNDED PRECEDING
345 ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
346 ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
347 The first two are equivalent and are also the default: they set the
349 frame to be all rows from the partition start up through the current
350 row's last peer in the ORDER BY ordering (which means all rows if there
351 is no ORDER BY). The options RANGE BETWEEN UNBOUNDED PRECEDING AND
352 UNBOUNDED FOLLOWING and ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED
353 FOLLOWING are also equivalent: they always select all rows in the par‐
354 tition. Lastly, ROWS UNBOUNDED PRECEDING or its verbose equivalent
355 ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW select all rows up
356 through the current row (regardless of duplicates). Beware that this
357 option can produce implementation-dependent results if the ORDER BY
358 ordering does not order the rows uniquely.
359 The purpose of a WINDOW clause is to specify the behavior of window
361 functions appearing in the query's SELECT List [select(7)] or ORDER BY
362 Clause [select(7)]. These functions can reference the WINDOW clause
363 entries by name in their OVER clauses. A WINDOW clause entry does not
364 have to be referenced anywhere, however; if it is not used in the query
365 it is simply ignored. It is possible to use window functions without
366 any WINDOW clause at all, since a window function call can specify its
367 window definition directly in its OVER clause. However, the WINDOW
368 clause saves typing when the same window definition is needed for more
369 than one window function.
370 Window functions are described in detail in in the documentation, in
372 the documentation, and in the documentation.
373 SELECT LIST
375 The SELECT list (between the key words SELECT and FROM) specifies
376 expressions that form the output rows of the SELECT statement. The
377 expressions can (and usually do) refer to columns computed in the FROM
378 clause.
379 Just as in a table, every output column of a SELECT has a name. In a
381 simple SELECT this name is just used to label the column for display,
382 but when the SELECT is a sub-query of a larger query, the name is seen
383 by the larger query as the column name of the virtual table produced by
384 the sub-query. To specify the name to use for an output column, write
385 AS output_name after the column's expression. (You can omit AS, but
386 only if the desired output name does not match any PostgreSQL keyword
387 (see in the documentation). For protection against possible future key‐
388 word additions, it is recommended that you always either write AS or
389 double-quote the output name.) If you do not specify a column name, a
390 name is chosen automatically by PostgreSQL. If the column's expression
391 is a simple column reference then the chosen name is the same as that
392 column's name; in more complex cases a generated name looking like
393 ?columnN? is usually chosen.
394 An output column's name can be used to refer to the column's value in
396 ORDER BY and GROUP BY clauses, but not in the WHERE or HAVING clauses;
397 there you must write out the expression instead.
398 Instead of an expression, * can be written in the output list as a
400 shorthand for all the columns of the selected rows. Also, you can write
401 table_name.* as a shorthand for the columns coming from just that ta‐
402 ble. In these cases it is not possible to specify new names with AS;
403 the output column names will be the same as the table columns' names.
404 UNION CLAUSE
406 The UNION clause has this general form:
407 select_statement UNION [ ALL ] select_statement
409 select_statement is any SELECT statement without an ORDER BY, LIMIT,
411 FOR UPDATE, or FOR SHARE clause. (ORDER BY and LIMIT can be attached
412 to a subexpression if it is enclosed in parentheses. Without parenthe‐
413 ses, these clauses will be taken to apply to the result of the UNION,
414 not to its right-hand input expression.)
415 The UNION operator computes the set union of the rows returned by the
417 involved SELECT statements. A row is in the set union of two result
418 sets if it appears in at least one of the result sets. The two SELECT
419 statements that represent the direct operands of the UNION must produce
420 the same number of columns, and corresponding columns must be of com‐
421 patible data types.
422 The result of UNION does not contain any duplicate rows unless the ALL
424 option is specified. ALL prevents elimination of duplicates. (There‐
425 fore, UNION ALL is usually significantly quicker than UNION; use ALL
426 when you can.)
427 Multiple UNION operators in the same SELECT statement are evaluated
429 left to right, unless otherwise indicated by parentheses.
430 Currently, FOR UPDATE and FOR SHARE cannot be specified either for a
432 UNION result or for any input of a UNION.
433 INTERSECT CLAUSE
435 The INTERSECT clause has this general form:
436 select_statement INTERSECT [ ALL ] select_statement
438 select_statement is any SELECT statement without an ORDER BY, LIMIT,
440 FOR UPDATE, or FOR SHARE clause.
441 The INTERSECT operator computes the set intersection of the rows
443 returned by the involved SELECT statements. A row is in the intersec‐
444 tion of two result sets if it appears in both result sets.
445 The result of INTERSECT does not contain any duplicate rows unless the
447 ALL option is specified. With ALL, a row that has m duplicates in the
448 left table and n duplicates in the right table will appear min(m,n)
449 times in the result set.
450 Multiple INTERSECT operators in the same SELECT statement are evaluated
452 left to right, unless parentheses dictate otherwise. INTERSECT binds
453 more tightly than UNION. That is, A UNION B INTERSECT C will be read as
454 A UNION (B INTERSECT C).
455 Currently, FOR UPDATE and FOR SHARE cannot be specified either for an
457 INTERSECT result or for any input of an INTERSECT.
458 EXCEPT CLAUSE
460 The EXCEPT clause has this general form:
461 select_statement EXCEPT [ ALL ] select_statement
463 select_statement is any SELECT statement without an ORDER BY, LIMIT,
465 FOR UPDATE, or FOR SHARE clause.
466 The EXCEPT operator computes the set of rows that are in the result of
468 the left SELECT statement but not in the result of the right one.
469 The result of EXCEPT does not contain any duplicate rows unless the ALL
471 option is specified. With ALL, a row that has m duplicates in the left
472 table and n duplicates in the right table will appear max(m-n,0) times
473 in the result set.
474 Multiple EXCEPT operators in the same SELECT statement are evaluated
476 left to right, unless parentheses dictate otherwise. EXCEPT binds at
477 the same level as UNION.
478 Currently, FOR UPDATE and FOR SHARE cannot be specified either for an
480 EXCEPT result or for any input of an EXCEPT.
481 ORDER BY CLAUSE
483 The optional ORDER BY clause has this general form:
484 ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]
486 The ORDER BY clause causes the result rows to be sorted according to
488 the specified expression(s). If two rows are equal according to the
489 leftmost expression, they are compared according to the next expression
490 and so on. If they are equal according to all specified expressions,
491 they are returned in an implementation-dependent order.
492 Each expression can be the name or ordinal number of an output column
494 (SELECT list item), or it can be an arbitrary expression formed from
495 input-column values.
496 The ordinal number refers to the ordinal (left-to-right) position of
498 the output column. This feature makes it possible to define an ordering
499 on the basis of a column that does not have a unique name. This is
500 never absolutely necessary because it is always possible to assign a
501 name to an output column using the AS clause.
502 It is also possible to use arbitrary expressions in the ORDER BY
504 clause, including columns that do not appear in the SELECT output list.
505 Thus the following statement is valid:
506 SELECT name FROM distributors ORDER BY code;
508 A limitation of this feature is that an ORDER BY clause applying to the
510 result of a UNION, INTERSECT, or EXCEPT clause can only specify an out‐
511 put column name or number, not an expression.
512 If an ORDER BY expression is a simple name that matches both an output
514 column name and an input column name, ORDER BY will interpret it as the
515 output column name. This is the opposite of the choice that GROUP BY
516 will make in the same situation. This inconsistency is made to be com‐
517 patible with the SQL standard.
518 Optionally one can add the key word ASC (ascending) or DESC (descend‐
520 ing) after any expression in the ORDER BY clause. If not specified, ASC
521 is assumed by default. Alternatively, a specific ordering operator name
522 can be specified in the USING clause. An ordering operator must be a
523 less-than or greater-than member of some B-tree operator family. ASC
524 is usually equivalent to USING < and DESC is usually equivalent to
525 USING >. (But the creator of a user-defined data type can define
526 exactly what the default sort ordering is, and it might correspond to
527 operators with other names.)
528 If NULLS LAST is specified, null values sort after all non-null values;
530 if NULLS FIRST is specified, null values sort before all non-null val‐
531 ues. If neither is specified, the default behavior is NULLS LAST when
532 ASC is specified or implied, and NULLS FIRST when DESC is specified
533 (thus, the default is to act as though nulls are larger than non-
534 nulls). When USING is specified, the default nulls ordering depends on
535 whether the operator is a less-than or greater-than operator.
536 Note that ordering options apply only to the expression they follow;
538 for example ORDER BY x, y DESC does not mean the same thing as ORDER BY
539 x DESC, y DESC.
540 Character-string data is sorted according to the locale-specific colla‐
542 tion order that was established when the database was created.
543 DISTINCT CLAUSE
545 If DISTINCT is specified, all duplicate rows are removed from the
546 result set (one row is kept from each group of duplicates). ALL speci‐
547 fies the opposite: all rows are kept; that is the default.
548 DISTINCT ON ( expression [, ...] ) keeps only the first row of each set
550 of rows where the given expressions evaluate to equal. The DISTINCT ON
551 expressions are interpreted using the same rules as for ORDER BY (see
552 above). Note that the ``first row'' of each set is unpredictable unless
553 ORDER BY is used to ensure that the desired row appears first. For
554 example:
555 SELECT DISTINCT ON (location) location, time, report
557 FROM weather_reports
558 ORDER BY location, time DESC;
559 retrieves the most recent weather report for each location. But if we
561 had not used ORDER BY to force descending order of time values for each
562 location, we'd have gotten a report from an unpredictable time for each
563 location.
564 The DISTINCT ON expression(s) must match the leftmost ORDER BY expres‐
566 sion(s). The ORDER BY clause will normally contain additional expres‐
567 sion(s) that determine the desired precedence of rows within each DIS‐
568 TINCT ON group.
569 LIMIT CLAUSE
571 The LIMIT clause consists of two independent sub-clauses:
572 LIMIT { count | ALL }
574 OFFSET start
575 count specifies the maximum number of rows to return, while start spec‐
577 ifies the number of rows to skip before starting to return rows. When
578 both are specified, start rows are skipped before starting to count the
579 count rows to be returned.
580 If the count expression evaluates to NULL, it is treated as LIMIT ALL,
582 i.e., no limit. If start evaluates to NULL, it is treated the same as
583 OFFSET 0.
584 SQL:2008 introduced a different syntax to achieve the same thing, which
586 PostgreSQL also supports. It is:
587 OFFSET start { ROW | ROWS }
589 FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
590 Both clauses are optional, but if present the OFFSET clause must come
592 before the FETCH clause. ROW and ROWS as well as FIRST and NEXT are
593 noise words that don't influence the effects of these clauses. In this
594 syntax, when using expressions other than simple constants for start or
595 count, parentheses will be necessary in most cases. If count is omitted
596 in FETCH, it defaults to 1.
597 When using LIMIT, it is a good idea to use an ORDER BY clause that con‐
599 strains the result rows into a unique order. Otherwise you will get an
600 unpredictable subset of the query's rows — you might be asking for the
601 tenth through twentieth rows, but tenth through twentieth in what
602 ordering? You don't know what ordering unless you specify ORDER BY.
603 The query planner takes LIMIT into account when generating a query
605 plan, so you are very likely to get different plans (yielding different
606 row orders) depending on what you use for LIMIT and OFFSET. Thus, using
607 different LIMIT/OFFSET values to select different subsets of a query
608 result will give inconsistent results unless you enforce a predictable
609 result ordering with ORDER BY. This is not a bug; it is an inherent
610 consequence of the fact that SQL does not promise to deliver the
611 results of a query in any particular order unless ORDER BY is used to
612 constrain the order.
613 It is even possible for repeated executions of the same LIMIT query to
615 return different subsets of the rows of a table, if there is not an
616 ORDER BY to enforce selection of a deterministic subset. Again, this is
617 not a bug; determinism of the results is simply not guaranteed in such
618 a case.
619 FOR UPDATE/FOR SHARE CLAUSE
621 The FOR UPDATE clause has this form:
622 FOR UPDATE [ OF table_name [, ...] ] [ NOWAIT ]
624 The closely related FOR SHARE clause has this form:
627 FOR SHARE [ OF table_name [, ...] ] [ NOWAIT ]
629 FOR UPDATE causes the rows retrieved by the SELECT statement to be
632 locked as though for update. This prevents them from being modified or
633 deleted by other transactions until the current transaction ends. That
634 is, other transactions that attempt UPDATE, DELETE, or SELECT FOR
635 UPDATE of these rows will be blocked until the current transaction
636 ends. Also, if an UPDATE, DELETE, or SELECT FOR UPDATE from another
637 transaction has already locked a selected row or rows, SELECT FOR
638 UPDATE will wait for the other transaction to complete, and will then
639 lock and return the updated row (or no row, if the row was deleted).
640 For further discussion see in the documentation.
641 To prevent the operation from waiting for other transactions to commit,
643 use the NOWAIT option. SELECT FOR UPDATE NOWAIT reports an error,
644 rather than waiting, if a selected row cannot be locked immediately.
645 Note that NOWAIT applies only to the row-level lock(s) — the required
646 ROW SHARE table-level lock is still taken in the ordinary way (see in
647 the documentation). You can use the NOWAIT option of LOCK [lock(7)] if
648 you need to acquire the table-level lock without waiting.
649 FOR SHARE behaves similarly, except that it acquires a shared rather
651 than exclusive lock on each retrieved row. A shared lock blocks other
652 transactions from performing UPDATE, DELETE, or SELECT FOR UPDATE on
653 these rows, but it does not prevent them from performing SELECT FOR
654 SHARE.
655 If specific tables are named in FOR UPDATE or FOR SHARE, then only rows
657 coming from those tables are locked; any other tables used in the
658 SELECT are simply read as usual. A FOR UPDATE or FOR SHARE clause with‐
659 out a table list affects all tables used in the command. If FOR UPDATE
660 or FOR SHARE is applied to a view or sub-query, it affects all tables
661 used in the view or sub-query. However, FOR UPDATE/FOR SHARE do not
662 apply to WITH queries referenced by the primary query. If you want row
663 locking to occur within a WITH query, specify FOR UPDATE or FOR SHARE
664 within the WITH query.
665 Multiple FOR UPDATE and FOR SHARE clauses can be written if it is nec‐
667 essary to specify different locking behavior for different tables. If
668 the same table is mentioned (or implicitly affected) by both FOR UPDATE
669 and FOR SHARE clauses, then it is processed as FOR UPDATE. Similarly, a
670 table is processed as NOWAIT if that is specified in any of the clauses
671 affecting it.
672 FOR UPDATE and FOR SHARE cannot be used in contexts where returned rows
674 cannot be clearly identified with individual table rows; for example
675 they cannot be used with aggregation.
676 Caution: Avoid locking a row and then modifying it within a
678 later savepoint or PL/pgSQL exception block. A subsequent roll‐
679 back would cause the lock to be lost. For example:
680 BEGIN;
682 SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
683 SAVEPOINT s;
684 UPDATE mytable SET ... WHERE key = 1;
685 ROLLBACK TO s;
686 After the ROLLBACK, the row is effectively unlocked, rather than
688 returned to its pre-savepoint state of being locked but not mod‐
689 ified. This hazard occurs if a row locked in the current trans‐
690 action is updated or deleted, or if a shared lock is upgraded to
691 exclusive: in all these cases, the former lock state is forgot‐
692 ten. If the transaction is then rolled back to a state between
693 the original locking command and the subsequent change, the row
694 will appear not to be locked at all. This is an implementation
695 deficiency which will be addressed in a future release of Post‐
696 greSQL.
697 Caution: It is possible for a SELECT command using both LIMIT
700 and FOR UPDATE/SHARE clauses to return fewer rows than specified
701 by LIMIT. This is because LIMIT is applied first. The command
702 selects the specified number of rows, but might then block try‐
703 ing to obtain a lock on one or more of them. Once the SELECT
704 unblocks, the row might have been deleted or updated so that it
705 does not meet the query WHERE condition anymore, in which case
706 it will not be returned.
707 Caution: Similarly, it is possible for a SELECT command using
710 ORDER BY and FOR UPDATE/SHARE to return rows out of order. This
711 is because ORDER BY is applied first. The command orders the
712 result, but might then block trying to obtain a lock on one or
713 more of the rows. Once the SELECT unblocks, one of the ordered
714 columns might have been modified and be returned out of order. A
715 workaround is to perform SELECT ... FOR UPDATE/SHARE and then
716 SELECT ... ORDER BY.
717 TABLE COMMAND
720 The command
721 TABLE name
723 is completely equivalent to
725 SELECT * FROM name
727 It can be used as a top-level command or as a space-saving syntax vari‐
729 ant in parts of complex queries.
730
732 To join the table films with the table distributors:
733 SELECT f.title, f.did, d.name, f.date_prod, f.kind
735 FROM distributors d, films f
736 WHERE f.did = d.did
737 title | did | name | date_prod | kind
739 -------------------+-----+--------------+------------+----------
740 The Third Man | 101 | British Lion | 1949-12-23 | Drama
741 The African Queen | 101 | British Lion | 1951-08-11 | Romantic
742 ...
743 To sum the column len of all films and group the results by kind:
746 SELECT kind, sum(len) AS total FROM films GROUP BY kind;
748 kind | total
750 ----------+-------
751 Action | 07:34
752 Comedy | 02:58
753 Drama | 14:28
754 Musical | 06:42
755 Romantic | 04:38
756 To sum the column len of all films, group the results by kind and show
759 those group totals that are less than 5 hours:
760 SELECT kind, sum(len) AS total
762 FROM films
763 GROUP BY kind
764 HAVING sum(len) < interval '5 hours';
765 kind | total
767 ----------+-------
768 Comedy | 02:58
769 Romantic | 04:38
770 The following two examples are identical ways of sorting the individual
773 results according to the contents of the second column (name):
774 SELECT * FROM distributors ORDER BY name;
776 SELECT * FROM distributors ORDER BY 2;
777 did | name
779 -----+------------------
780 109 | 20th Century Fox
781 110 | Bavaria Atelier
782 101 | British Lion
783 107 | Columbia
784 102 | Jean Luc Godard
785 113 | Luso films
786 104 | Mosfilm
787 103 | Paramount
788 106 | Toho
789 105 | United Artists
790 111 | Walt Disney
791 112 | Warner Bros.
792 108 | Westward
793 The next example shows how to obtain the union of the tables distribu‐
796 tors and actors, restricting the results to those that begin with the
797 letter W in each table. Only distinct rows are wanted, so the key word
798 ALL is omitted.
799 distributors: actors:
801 did | name id | name
802 -----+-------------- ----+----------------
803 108 | Westward 1 | Woody Allen
804 111 | Walt Disney 2 | Warren Beatty
805 112 | Warner Bros. 3 | Walter Matthau
806 ... ...
807 SELECT distributors.name
809 FROM distributors
810 WHERE distributors.name LIKE 'W%'
811 UNION
812 SELECT actors.name
813 FROM actors
814 WHERE actors.name LIKE 'W%';
815 name
817 ----------------
818 Walt Disney
819 Walter Matthau
820 Warner Bros.
821 Warren Beatty
822 Westward
823 Woody Allen
824 This example shows how to use a function in the FROM clause, both with
827 and without a column definition list:
828 CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
830 SELECT * FROM distributors WHERE did = $1;
831 $$ LANGUAGE SQL;
832 SELECT * FROM distributors(111);
834 did | name
835 -----+-------------
836 111 | Walt Disney
837 CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
839 SELECT * FROM distributors WHERE did = $1;
840 $$ LANGUAGE SQL;
841 SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
843 f1 | f2
844 -----+-------------
845 111 | Walt Disney
846 This example shows how to use a simple WITH clause:
849 WITH t AS (
851 SELECT random() as x FROM generate_series(1, 3)
852 )
853 SELECT * FROM t
854 UNION ALL
855 SELECT * FROM t
856 x
858 --------------------
859 0.534150459803641
860 0.520092216785997
861 0.0735620250925422
862 0.534150459803641
863 0.520092216785997
864 0.0735620250925422
865 Notice that the WITH query was evaluated only once, so that we got two
867 sets of the same three random values.
868 This example uses WITH RECURSIVE to find all subordinates (direct or
870 indirect) of the employee Mary, and their level of indirectness, from a
871 table that shows only direct subordinates:
872 WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
874 SELECT 1, employee_name, manager_name
875 FROM employee
876 WHERE manager_name = 'Mary'
877 UNION ALL
878 SELECT er.distance + 1, e.employee_name, e.manager_name
879 FROM employee_recursive er, employee e
880 WHERE er.employee_name = e.manager_name
881 )
882 SELECT distance, employee_name FROM employee_recursive;
883 Notice the typical form of recursive queries: an initial condition,
885 followed by UNION, followed by the recursive part of the query. Be sure
886 that the recursive part of the query will eventually return no tuples,
887 or else the query will loop indefinitely. (See in the documentation for
888 more examples.)
889
891 Of course, the SELECT statement is compatible with the SQL standard.
892 But there are some extensions and some missing features.
893 OMITTED FROM CLAUSES
895 PostgreSQL allows one to omit the FROM clause. It has a straightforward
896 use to compute the results of simple expressions:
897 SELECT 2+2;
899 ?column?
901 ----------
902 4
903 Some other SQL databases cannot do this except by introducing a dummy
905 one-row table from which to do the SELECT.
906 Note that if a FROM clause is not specified, the query cannot reference
908 any database tables. For example, the following query is invalid:
909 SELECT distributors.* WHERE distributors.name = 'Westward';
911 PostgreSQL releases prior to 8.1 would accept queries of this form, and
913 add an implicit entry to the query's FROM clause for each table refer‐
914 enced by the query. This is no longer the default behavior, because it
915 does not comply with the SQL standard, and is considered by many to be
916 error-prone. For compatibility with applications that rely on this
917 behavior the add_missing_from configuration variable can be enabled.
918 OMITTING THE AS KEY WORD
920 In the SQL standard, the optional key word AS can be omitted before an
921 output column name whenever the new column name is a valid column name
922 (that is, not the same as any reserved keyword). PostgreSQL is slightly
923 more restrictive: AS is required if the new column name matches any
924 keyword at all, reserved or not. Recommended practice is to use AS or
925 double-quote output column names, to prevent any possible conflict
926 against future keyword additions.
927 In FROM items, both the standard and PostgreSQL allow AS to be omitted
929 before an alias that is an unreserved keyword. But this is impractical
930 for output column names, because of syntactic ambiguities.
931 ONLY AND INHERITANCE
933 The SQL standard requires parentheses around the table name when writ‐
934 ing ONLY, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE ....
935 PostgreSQL considers these parentheses to be optional.
936 PostgreSQL allows a trailing * to be written to explicitly specify the
938 non-ONLY behavior of including child tables. The standard does not
939 allow this.
940 (These points apply equally to all SQL commands supporting the ONLY
942 option.)
943 NAMESPACE AVAILABLE TO GROUP BY AND ORDER BY
945 In the SQL-92 standard, an ORDER BY clause can only use output column
946 names or numbers, while a GROUP BY clause can only use expressions
947 based on input column names. PostgreSQL extends each of these clauses
948 to allow the other choice as well (but it uses the standard's interpre‐
949 tation if there is ambiguity). PostgreSQL also allows both clauses to
950 specify arbitrary expressions. Note that names appearing in an expres‐
951 sion will always be taken as input-column names, not as output-column
952 names.
953 SQL:1999 and later use a slightly different definition which is not
955 entirely upward compatible with SQL-92. In most cases, however, Post‐
956 greSQL will interpret an ORDER BY or GROUP BY expression the same way
957 SQL:1999 does.
958 WINDOW CLAUSE RESTRICTIONS
960 The SQL standard provides additional options for the window
961 frame_clause. PostgreSQL currently supports only the options listed
962 above.
963 LIMIT AND OFFSET
965 The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used
966 by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ...
967 FETCH {FIRST|NEXT} ... for the same functionality, as shown above in
968 LIMIT Clause [select(7)], and this syntax is also used by IBM DB2.
969 (Applications written for Oracle frequently use a workaround involving
970 the automatically generated rownum column, not available in PostgreSQL,
971 to implement the effects of these clauses.)
972 NONSTANDARD CLAUSES
974 The clause DISTINCT ON is not defined in the SQL standard.
975SQL - Language Statements 2014-02-17 SELECT(7)