Name
SELECT, TABLE, WITH -- retrieve rows from a table or viewSynopsis
[ WITH [ RECURSIVE ] with_query [, ...] ] SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ] [ * | expression [ [ AS ] output_name ] [, ...] ] [ FROM from_item [, ...] ] [ WHERE condition ] [ GROUP BY grouping_element [, ...] ] [ HAVING condition [, ...] ] [ WINDOW window_name AS ( window_definition ) [, ...] ] [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ] [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ] [ LIMIT { count | ALL } ] [ OFFSET start [ ROW | ROWS ] ] [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ] [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ] where from_item can be one of: [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ] [ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ] [ LATERAL ] ( select ) [ AS ] alias [ ( column_alias [, ...] ) ] with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ] [ LATERAL ] function_name ( [ argument [, ...] ] ) [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ] [ LATERAL ] function_name ( [ argument [, ...] ] ) [ AS ] alias ( column_definition [, ...] ) [ LATERAL ] function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] ) [ LATERAL ] ROWS FROM( function_name ( [ argument [, ...] ] ) [ AS ( column_definition [, ...] ) ] [, ...] ) [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ] from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ] and grouping_element can be one of: ( ) expression ( expression [, ...] ) ROLLUP ( { expression | ( expression [, ...] ) } [, ...] ) CUBE ( { expression | ( expression [, ...] ) } [, ...] ) GROUPING SETS ( grouping_element [, ...] ) and with_query is: with_query_name [ ( column_name [, ...] ) ] AS ( select | values | insert | update | delete ) TABLE [ ONLY ] table_name [ * ]
Description
SELECT
retrieves rows from zero or more tables. The general processing of SELECT
is as follows:
-
All queries in the
WITH
list are computed. These effectively serve as temporary tables that can be referenced in theFROM
list. AWITH
query that is referenced more than once inFROM
is computed only once. (See WITH Clause below.) -
All elements in the
FROM
list are computed. (Each element in theFROM
list is a real or virtual table.) If more than one element is specified in theFROM
list, they are cross-joined together. (See FROM Clause below.) -
If the
WHERE
clause is specified, all rows that do not satisfy the condition are eliminated from the output. (See WHERE Clause below.) -
If the
GROUP BY
clause is specified, or if there are aggregate function calls, the output is combined into groups of rows that match on one or more values, and the results of aggregate functions are computed. If theHAVING
clause is present, it eliminates groups that do not satisfy the given condition. (See GROUP BY Clause and HAVING Clause below.) -
The actual output rows are computed using the
SELECT
output expressions for each selected row or row group. (See SELECT List below.) -
SELECT DISTINCT
eliminates duplicate rows from the result.SELECT DISTINCT ON
eliminates rows that match on all the specified expressions.SELECT ALL
(the default) will return all candidate rows, including duplicates. (See DISTINCT Clause below.) -
Using the operators
UNION
,INTERSECT
, andEXCEPT
, the output of more than oneSELECT
statement can be combined to form a single result set. TheUNION
operator returns all rows that are in one or both of the result sets. TheINTERSECT
operator returns all rows that are strictly in both result sets. TheEXCEPT
operator returns the rows that are in the first result set but not in the second. In all three cases, duplicate rows are eliminated unlessALL
is specified. The noise wordDISTINCT
can be added to explicitly specify eliminating duplicate rows. Notice thatDISTINCT
is the default behavior here, even thoughALL
is the default forSELECT
itself. (See UNION Clause, INTERSECT Clause, and EXCEPT Clause below.) -
If the
ORDER BY
clause is specified, the returned rows are sorted in the specified order. IfORDER BY
is not given, the rows are returned in whatever order the system finds fastest to produce. (See ORDER BY Clause below.) -
If the
LIMIT
(orFETCH FIRST
) orOFFSET
clause is specified, theSELECT
statement only returns a subset of the result rows. (See LIMIT Clause below.) -
If
FOR UPDATE
,FOR NO KEY UPDATE
,FOR SHARE
orFOR KEY SHARE
is specified, theSELECT
statement locks the selected rows against concurrent updates. (See The Locking Clause below.)
You must have SELECT
privilege on each column used in a SELECT
command. The use of FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
or FOR KEY SHARE
requires UPDATE
privilege as well (for at least one column of each table so selected).
Parameters
WITH
Clause
The WITH
clause allows you to specify one or more subqueries that can be referenced by name in the primary query. The subqueries effectively act as temporary tables or views for the duration of the primary query. Each subquery can be a SELECT
, TABLE
, VALUES
, INSERT
, UPDATE
or DELETE
statement. When writing a data-modifying statement (INSERT
, UPDATE
or DELETE
) in WITH
, it is usual to include a RETURNING
clause. It is the output of RETURNING
, not the underlying table that the statement modifies, that forms the temporary table that is read by the primary query. If RETURNING
is omitted, the statement is still executed, but it produces no output so it cannot be referenced as a table by the primary query.
A name (without schema qualification) must be specified for each WITH
query. Optionally, a list of column names can be specified; if this is omitted, the column names are inferred from the subquery.
If RECURSIVE
is specified, it allows a SELECT
subquery to reference itself by name. Such a subquery must have the form
non_recursive_term UNION [ ALL | DISTINCT ] recursive_term
where the recursive self-reference must appear on the right-hand side of the UNION
. Only one recursive self-reference is permitted per query. Recursive data-modifying statements are not supported, but you can use the results of a recursive SELECT
query in a data-modifying statement. See Section 7.8 for an example.
Another effect of RECURSIVE
is that WITH
queries need not be ordered: a query can reference another one that is later in the list. (However, circular references, or mutual recursion, are not implemented.) Without RECURSIVE
, WITH
queries can only reference sibling WITH
queries that are earlier in the WITH
list.
A key property of WITH
queries is that they are evaluated only once per execution of the primary query, even if the primary query refers to them more than once. In particular, data-modifying statements are guaranteed to be executed once and only once, regardless of whether the primary query reads all or any of their output.
The primary query and the WITH
queries are all (notionally) executed at the same time. This implies that the effects of a data-modifying statement in WITH
cannot be seen from other parts of the query, other than by reading its RETURNING
output. If two such data-modifying statements attempt to modify the same row, the results are unspecified.
See Section 7.8 for additional information.
FROM
Clause
The FROM
clause specifies one or more source tables for the SELECT
. If multiple sources are specified, the result is the Cartesian product (cross join) of all the sources. But usually qualification conditions are added (via WHERE
) to restrict the returned rows to a small subset of the Cartesian product.
The FROM
clause can contain the following elements:
table_name
-
The name (optionally schema-qualified) of an existing table or view. If
ONLY
is specified before the table name, only that table is scanned. IfONLY
is not specified, the table and all its descendant tables (if any) are scanned. Optionally,*
can be specified after the table name to explicitly indicate that descendant tables are included. alias
-
A substitute name for the
FROM
item containing the alias. An alias is used for brevity or to eliminate ambiguity for self-joins (where the same table is scanned multiple times). When an alias is provided, it completely hides the actual name of the table or function; for example givenFROM foo AS f
, the remainder of theSELECT
must refer to thisFROM
item asf
notfoo
. If an alias is written, a column alias list can also be written to provide substitute names for one or more columns of the table. TABLESAMPLE
sampling_method
(argument
[, ...] ) [ REPEATABLE (seed
) ]-
A
TABLESAMPLE
clause after atable_name
indicates that the specifiedsampling_method
should be used to retrieve a subset of the rows in that table. This sampling precedes the application of any other filters such asWHERE
clauses. The standard PostgreSQL distribution includes two sampling methods,BERNOULLI
andSYSTEM
, and other sampling methods can be installed in the database via extensions.The
BERNOULLI
andSYSTEM
sampling methods each accept a singleargument
which is the fraction of the table to sample, expressed as a percentage between 0 and 100. This argument can be anyreal
-valued expression. (Other sampling methods might accept more or different arguments.) These two methods each return a randomly-chosen sample of the table that will contain approximately the specified percentage of the table's rows. TheBERNOULLI
method scans the whole table and selects or ignores individual rows independently with the specified probability. TheSYSTEM
method does block-level sampling with each block having the specified chance of being selected; all rows in each selected block are returned. TheSYSTEM
method is significantly faster than theBERNOULLI
method when small sampling percentages are specified, but it may return a less-random sample of the table as a result of clustering effects.The optional
REPEATABLE
clause specifies aseed
number or expression to use for generating random numbers within the sampling method. The seed value can be any non-null floating-point value. Two queries that specify the same seed andargument
values will select the same sample of the table, if the table has not been changed meanwhile. But different seed values will usually produce different samples. IfREPEATABLE
is not given then a new random sample is selected for each query, based upon a system-generated seed. Note that some add-on sampling methods do not acceptREPEATABLE
, and will always produce new samples on each use. select
-
A sub-
SELECT
can appear in theFROM
clause. This acts as though its output were created as a temporary table for the duration of this singleSELECT
command. Note that the sub-SELECT
must be surrounded by parentheses, and an alias must be provided for it. A VALUES command can also be used here. with_query_name
-
A
WITH
query is referenced by writing its name, just as though the query's name were a table name. (In fact, theWITH
query hides any real table of the same name for the purposes of the primary query. If necessary, you can refer to a real table of the same name by schema-qualifying the table's name.) An alias can be provided in the same way as for a table. function_name
-
Function calls can appear in the
FROM
clause. (This is especially useful for functions that return result sets, but any function can be used.) This acts as though the function's output were created as a temporary table for the duration of this singleSELECT
command. When the optionalWITH ORDINALITY
clause is added to the function call, a new column is appended after all the function's output columns with numbering for each row.An alias can be provided in the same way as for a table. If an alias is written, a column alias list can also be written to provide substitute names for one or more attributes of the function's composite return type, including the column added by
ORDINALITY
if present.Multiple function calls can be combined into a single
FROM
-clause item by surrounding them withROWS FROM( ... )
. The output of such an item is the concatenation of the first row from each function, then the second row from each function, etc. If some of the functions produce fewer rows than others, null values are substituted for the missing data, so that the total number of rows returned is always the same as for the function that produced the most rows.If the function has been defined as returning the
record
data type, then an alias or the key wordAS
must be present, followed by a column definition list in the form(
. The column definition list must match the actual number and types of columns returned by the function.column_name
data_type
[, ... ])When using the
ROWS FROM( ... )
syntax, if one of the functions requires a column definition list, it's preferred to put the column definition list after the function call insideROWS FROM( ... )
. A column definition list can be placed after theROWS FROM( ... )
construct only if there's just a single function and noWITH ORDINALITY
clause.To use
ORDINALITY
together with a column definition list, you must use theROWS FROM( ... )
syntax and put the column definition list insideROWS FROM( ... )
. join_type
-
One of
-
[ INNER ] JOIN
-
LEFT [ OUTER ] JOIN
-
RIGHT [ OUTER ] JOIN
-
FULL [ OUTER ] JOIN
-
CROSS JOIN
For the
INNER
andOUTER
join types, a join condition must be specified, namely exactly one ofNATURAL
,ON
, orjoin_condition
USING (
. See below for the meaning. Forjoin_column
[, ...])CROSS JOIN
, none of these clauses can appear.A
JOIN
clause combines twoFROM
items, which for convenience we will refer to as "tables", though in reality they can be any type ofFROM
item. Use parentheses if necessary to determine the order of nesting. In the absence of parentheses,JOIN
s nest left-to-right. In any caseJOIN
binds more tightly than the commas separatingFROM
-list items.CROSS JOIN
andINNER JOIN
produce a simple Cartesian product, the same result as you get from listing the two tables at the top level ofFROM
, but restricted by the join condition (if any).CROSS JOIN
is equivalent toINNER JOIN ON (TRUE)
, that is, no rows are removed by qualification. These join types are just a notational convenience, since they do nothing you couldn't do with plainFROM
andWHERE
.LEFT OUTER JOIN
returns all rows in the qualified Cartesian product (i.e., all combined rows that pass its join condition), plus one copy of each row in the left-hand table for which there was no right-hand row that passed the join condition. This left-hand row is extended to the full width of the joined table by inserting null values for the right-hand columns. Note that only theJOIN
clause's own condition is considered while deciding which rows have matches. Outer conditions are applied afterwards.Conversely,
RIGHT OUTER JOIN
returns all the joined rows, plus one row for each unmatched right-hand row (extended with nulls on the left). This is just a notational convenience, since you could convert it to aLEFT OUTER JOIN
by switching the left and right tables.FULL OUTER JOIN
returns all the joined rows, plus one row for each unmatched left-hand row (extended with nulls on the right), plus one row for each unmatched right-hand row (extended with nulls on the left). -
ON
join_condition
-
join_condition
is an expression resulting in a value of typeboolean
(similar to aWHERE
clause) that specifies which rows in a join are considered to match. USING (
join_column
[, ...] )-
A clause of the form
USING ( a, b, ... )
is shorthand forON left_table.a = right_table.a AND left_table.b = right_table.b ...
. Also,USING
implies that only one of each pair of equivalent columns will be included in the join output, not both. NATURAL
-
NATURAL
is shorthand for aUSING
list that mentions all columns in the two tables that have the same names. LATERAL
-
The
LATERAL
key word can precede a sub-SELECT
FROM
item. This allows the sub-SELECT
to refer to columns ofFROM
items that appear before it in theFROM
list. (WithoutLATERAL
, each sub-SELECT
is evaluated independently and so cannot cross-reference any otherFROM
item.)LATERAL
can also precede a function-callFROM
item, but in this case it is a noise word, because the function expression can refer to earlierFROM
items in any case.A
LATERAL
item can appear at top level in theFROM
list, or within aJOIN
tree. In the latter case it can also refer to any items that are on the left-hand side of aJOIN
that it is on the right-hand side of.When a
FROM
item containsLATERAL
cross-references, evaluation proceeds as follows: for each row of theFROM
item providing the cross-referenced column(s), or set of rows of multipleFROM
items providing the columns, theLATERAL
item is evaluated using that row or row set's values of the columns. The resulting row(s) are joined as usual with the rows they were computed from. This is repeated for each row or set of rows from the column source table(s).The column source table(s) must be
INNER
orLEFT
joined to theLATERAL
item, else there would not be a well-defined set of rows from which to compute each set of rows for theLATERAL
item. Thus, although a construct such as
is syntactically valid, it is not actually allowed forX
RIGHT JOIN LATERALY
Y
to referenceX
.
WHERE
Clause
The optional WHERE
clause has the general form
WHERE condition
where condition
is any expression that evaluates to a result of type boolean
. Any row that does not satisfy this condition will be eliminated from the output. A row satisfies the condition if it returns true when the actual row values are substituted for any variable references.
GROUP BY
Clause
The optional GROUP BY
clause has the general form
GROUP BY grouping_element [, ...]
GROUP BY
will condense into a single row all selected rows that share the same values for the grouped expressions. An expression
used inside a grouping_element
can be an input column name, or the name or ordinal number of an output column (SELECT
list item), or an arbitrary expression formed from input-column values. In case of ambiguity, a GROUP BY
name will be interpreted as an input-column name rather than an output column name.
If any of GROUPING SETS
, ROLLUP
or CUBE
are present as grouping elements, then the GROUP BY
clause as a whole defines some number of independent grouping sets
. The effect of this is equivalent to constructing a UNION ALL
between subqueries with the individual grouping sets as their GROUP BY
clauses. For further details on the handling of grouping sets see Section 7.2.4.
Aggregate functions, if any are used, are computed across all rows making up each group, producing a separate value for each group. (If there are aggregate functions but no GROUP BY
clause, the query is treated as having a single group comprising all the selected rows.) The set of rows fed to each aggregate function can be further filtered by attaching a FILTER
clause to the aggregate function call; see Section 4.2.7 for more information. When a FILTER
clause is present, only those rows matching it are included in the input to that aggregate function.
When GROUP BY
is present, or any aggregate functions are present, it is not valid for the SELECT
list expressions to refer to ungrouped columns except within aggregate functions or when the ungrouped column is functionally dependent on the grouped columns, since there would otherwise be more than one possible value to return for an ungrouped column. A functional dependency exists if the grouped columns (or a subset thereof) are the primary key of the table containing the ungrouped column.
Keep in mind that all aggregate functions are evaluated before evaluating any "scalar" expressions in the HAVING
clause or SELECT
list. This means that, for example, a CASE
expression cannot be used to skip evaluation of an aggregate function; see Section 4.2.14.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified with GROUP BY
.
HAVING
Clause
The optional HAVING
clause has the general form
HAVING condition
where condition
is the same as specified for the WHERE
clause.
HAVING
eliminates group rows that do not satisfy the condition. HAVING
is different from WHERE
: WHERE
filters individual rows before the application of GROUP BY
, while HAVING
filters group rows created by GROUP BY
. Each column referenced in condition
must unambiguously reference a grouping column, unless the reference appears within an aggregate function or the ungrouped column is functionally dependent on the grouping columns.
The presence of HAVING
turns a query into a grouped query even if there is no GROUP BY
clause. This is the same as what happens when the query contains aggregate functions but no GROUP BY
clause. All the selected rows are considered to form a single group, and the SELECT
list and HAVING
clause can only reference table columns from within aggregate functions. Such a query will emit a single row if the HAVING
condition is true, zero rows if it is not true.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified with HAVING
.
WINDOW
Clause
The optional WINDOW
clause has the general form
WINDOW window_name AS ( window_definition ) [, ...]
where window_name
is a name that can be referenced from OVER
clauses or subsequent window definitions, and window_definition
is
[ existing_window_name ] [ PARTITION BY expression [, ...] ] [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ] [ frame_clause ]
If an existing_window_name
is specified it must refer to an earlier entry in the WINDOW
list; the new window copies its partitioning clause from that entry, as well as its ordering clause if any. In this case the new window cannot specify its own PARTITION BY
clause, and it can specify ORDER BY
only if the copied window does not have one. The new window always uses its own frame clause; the copied window must not specify a frame clause.
The elements of the PARTITION BY
list are interpreted in much the same fashion as elements of a GROUP BY Clause, except that they are always simple expressions and never the name or number of an output column. Another difference is that these expressions can contain aggregate function calls, which are not allowed in a regular GROUP BY
clause. They are allowed here because windowing occurs after grouping and aggregation.
Similarly, the elements of the ORDER BY
list are interpreted in much the same fashion as elements of an ORDER BY Clause, except that the expressions are always taken as simple expressions and never the name or number of an output column.
The optional frame_clause
defines the window frame for window functions that depend on the frame (not all do). The window frame is a set of related rows for each row of the query (called the current row). The frame_clause
can be one of
{ RANGE | ROWS } frame_start { RANGE | ROWS } BETWEEN frame_start AND frame_end
where frame_start
and frame_end
can be one of
UNBOUNDED PRECEDING value PRECEDING CURRENT ROW value FOLLOWING UNBOUNDED FOLLOWING
If frame_end
is omitted it defaults to CURRENT ROW
. Restrictions are that frame_start
cannot be UNBOUNDED FOLLOWING
, frame_end
cannot be UNBOUNDED PRECEDING
, and the frame_end
choice cannot appear earlier in the above list than the frame_start
choice — for example RANGE BETWEEN CURRENT ROW AND
is not allowed.value
PRECEDING
The default framing option is RANGE UNBOUNDED PRECEDING
, which is the same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
; it sets the frame to be all rows from the partition start up through the current row's last peer (a row that ORDER BY
considers equivalent to the current row, or all rows if there is no ORDER BY
). In general, UNBOUNDED PRECEDING
means that the frame starts with the first row of the partition, and similarly UNBOUNDED FOLLOWING
means that the frame ends with the last row of the partition (regardless of RANGE
or ROWS
mode). In ROWS
mode, CURRENT ROW
means that the frame starts or ends with the current row; but in RANGE
mode it means that the frame starts or ends with the current row's first or last peer in the ORDER BY
ordering. The value
PRECEDING
and value
FOLLOWING
cases are currently only allowed in ROWS
mode. They indicate that the frame starts or ends with the row that many rows before or after the current row. value
must be an integer expression not containing any variables, aggregate functions, or window functions. The value must not be null or negative; but it can be zero, which selects the current row itself.
Beware that the ROWS
options can produce unpredictable results if the ORDER BY
ordering does not order the rows uniquely. The RANGE
options are designed to ensure that rows that are peers in the ORDER BY
ordering are treated alike; all peer rows will be in the same frame.
The purpose of a WINDOW
clause is to specify the behavior of window functions appearing in the query's SELECT List or ORDER BY Clause. These functions can reference the WINDOW
clause entries by name in their OVER
clauses. A WINDOW
clause entry does not have to be referenced anywhere, however; if it is not used in the query it is simply ignored. It is possible to use window functions without any WINDOW
clause at all, since a window function call can specify its window definition directly in its OVER
clause. However, the WINDOW
clause saves typing when the same window definition is needed for more than one window function.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified with WINDOW
.
Window functions are described in detail in Section 3.5, Section 4.2.8, and Section 7.2.5.
SELECT
List
The SELECT
list (between the key words SELECT
and FROM
) specifies expressions that form the output rows of the SELECT
statement. The expressions can (and usually do) refer to columns computed in the FROM
clause.
Just as in a table, every output column of a SELECT
has a name. In a simple SELECT
this name is just used to label the column for display, but when the SELECT
is a sub-query of a larger query, the name is seen by the larger query as the column name of the virtual table produced by the sub-query. To specify the name to use for an output column, write AS
output_name
after the column's expression. (You can omit AS
, but only if the desired output name does not match any PostgreSQL keyword (see Appendix C). For protection against possible future keyword additions, it is recommended that you always either write AS
or double-quote the output name.) If you do not specify a column name, a name is chosen automatically by PostgreSQL. If the column's expression is a simple column reference then the chosen name is the same as that column's name. In more complex cases a function or type name may be used, or the system may fall back on a generated name such as ?column?
.
An output column's name can be used to refer to the column's value in ORDER BY
and GROUP BY
clauses, but not in the WHERE
or HAVING
clauses; there you must write out the expression instead.
Instead of an expression, *
can be written in the output list as a shorthand for all the columns of the selected rows. Also, you can write
as a shorthand for the columns coming from just that table. In these cases it is not possible to specify new names with table_name
.*AS
; the output column names will be the same as the table columns' names.
According to the SQL standard, the expressions in the output list should be computed before applying DISTINCT
, ORDER BY
, or LIMIT
. This is obviously necessary when using DISTINCT
, since otherwise it's not clear what values are being made distinct. However, in many cases it is convenient if output expressions are computed after ORDER BY
and LIMIT
; particularly if the output list contains any volatile or expensive functions. With that behavior, the order of function evaluations is more intuitive and there will not be evaluations corresponding to rows that never appear in the output. PostgreSQL will effectively evaluate output expressions after sorting and limiting, so long as those expressions are not referenced in DISTINCT
, ORDER BY
or GROUP BY
. (As a counterexample, SELECT f(x) FROM tab ORDER BY 1
clearly must evaluate f(x)
before sorting.) Output expressions that contain set-returning functions are effectively evaluated after sorting and before limiting, so that LIMIT
will act to cut off the output from a set-returning function.
Note: PostgreSQL versions before 9.6 did not provide any guarantees about the timing of evaluation of output expressions versus sorting and limiting; it depended on the form of the chosen query plan.
DISTINCT
Clause
If SELECT DISTINCT
is specified, all duplicate rows are removed from the result set (one row is kept from each group of duplicates). SELECT ALL
specifies the opposite: all rows are kept; that is the default.
SELECT DISTINCT ON (
keeps only the first row of each set of rows where the given expressions evaluate to equal. The expression
[, ...] )DISTINCT ON
expressions are interpreted using the same rules as for ORDER BY
(see above). Note that the "first row" of each set is unpredictable unless ORDER BY
is used to ensure that the desired row appears first. For example:
SELECT DISTINCT ON (location) location, time, report FROM weather_reports ORDER BY location, time DESC;
retrieves the most recent weather report for each location. But if we had not used ORDER BY
to force descending order of time values for each location, we'd have gotten a report from an unpredictable time for each location.
The DISTINCT ON
expression(s) must match the leftmost ORDER BY
expression(s). The ORDER BY
clause will normally contain additional expression(s) that determine the desired precedence of rows within each DISTINCT ON
group.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified with DISTINCT
.
UNION
Clause
The UNION
clause has this general form:
select_statement UNION [ ALL | DISTINCT ] select_statement
select_statement
is any SELECT
statement without an ORDER BY
, LIMIT
, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
, or FOR KEY SHARE
clause. (ORDER BY
and LIMIT
can be attached to a subexpression if it is enclosed in parentheses. Without parentheses, these clauses will be taken to apply to the result of the UNION
, not to its right-hand input expression.)
The UNION
operator computes the set union of the rows returned by the involved SELECT
statements. A row is in the set union of two result sets if it appears in at least one of the result sets. The two SELECT
statements that represent the direct operands of the UNION
must produce the same number of columns, and corresponding columns must be of compatible data types.
The result of UNION
does not contain any duplicate rows unless the ALL
option is specified. ALL
prevents elimination of duplicates. (Therefore, UNION ALL
is usually significantly quicker than UNION
; use ALL
when you can.) DISTINCT
can be written to explicitly specify the default behavior of eliminating duplicate rows.
Multiple UNION
operators in the same SELECT
statement are evaluated left to right, unless otherwise indicated by parentheses.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified either for a UNION
result or for any input of a UNION
.
INTERSECT
Clause
The INTERSECT
clause has this general form:
select_statement INTERSECT [ ALL | DISTINCT ] select_statement
select_statement
is any SELECT
statement without an ORDER BY
, LIMIT
, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
, or FOR KEY SHARE
clause.
The INTERSECT
operator computes the set intersection of the rows returned by the involved SELECT
statements. A row is in the intersection of two result sets if it appears in both result sets.
The result of INTERSECT
does not contain any duplicate rows unless the ALL
option is specified. With ALL
, a row that has m
duplicates in the left table and n
duplicates in the right table will appear min(m
,n
) times in the result set. DISTINCT
can be written to explicitly specify the default behavior of eliminating duplicate rows.
Multiple INTERSECT
operators in the same SELECT
statement are evaluated left to right, unless parentheses dictate otherwise. INTERSECT
binds more tightly than UNION
. That is, A UNION B INTERSECT C
will be read as A UNION (B INTERSECT C)
.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified either for an INTERSECT
result or for any input of an INTERSECT
.
EXCEPT
Clause
The EXCEPT
clause has this general form:
select_statement EXCEPT [ ALL | DISTINCT ] select_statement
select_statement
is any SELECT
statement without an ORDER BY
, LIMIT
, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
, or FOR KEY SHARE
clause.
The EXCEPT
operator computes the set of rows that are in the result of the left SELECT
statement but not in the result of the right one.
The result of EXCEPT
does not contain any duplicate rows unless the ALL
option is specified. With ALL
, a row that has m
duplicates in the left table and n
duplicates in the right table will appear max(m
-n
,0) times in the result set. DISTINCT
can be written to explicitly specify the default behavior of eliminating duplicate rows.
Multiple EXCEPT
operators in the same SELECT
statement are evaluated left to right, unless parentheses dictate otherwise. EXCEPT
binds at the same level as UNION
.
Currently, FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
and FOR KEY SHARE
cannot be specified either for an EXCEPT
result or for any input of an EXCEPT
.
ORDER BY
Clause
The optional ORDER BY
clause has this general form:
ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...]
The ORDER BY
clause causes the result rows to be sorted according to the specified expression(s). If two rows are equal according to the leftmost expression, they are compared according to the next expression and so on. If they are equal according to all specified expressions, they are returned in an implementation-dependent order.
Each expression
can be the name or ordinal number of an output column (SELECT
list item), or it can be an arbitrary expression formed from input-column values.
The ordinal number refers to the ordinal (left-to-right) position of the output column. This feature makes it possible to define an ordering on the basis of a column that does not have a unique name. This is never absolutely necessary because it is always possible to assign a name to an output column using the AS
clause.
It is also possible to use arbitrary expressions in the ORDER BY
clause, including columns that do not appear in the SELECT
output list. Thus the following statement is valid:
SELECT name FROM distributors ORDER BY code;
A limitation of this feature is that an ORDER BY
clause applying to the result of a UNION
, INTERSECT
, or EXCEPT
clause can only specify an output column name or number, not an expression.
If an ORDER BY
expression is a simple name that matches both an output column name and an input column name, ORDER BY
will interpret it as the output column name. This is the opposite of the choice that GROUP BY
will make in the same situation. This inconsistency is made to be compatible with the SQL standard.
Optionally one can add the key word ASC
(ascending) or DESC
(descending) after any expression in the ORDER BY
clause. If not specified, ASC
is assumed by default. Alternatively, a specific ordering operator name can be specified in the USING
clause. An ordering operator must be a less-than or greater-than member of some B-tree operator family. ASC
is usually equivalent to USING <
and DESC
is usually equivalent to USING >
. (But the creator of a user-defined data type can define exactly what the default sort ordering is, and it might correspond to operators with other names.)
If NULLS LAST
is specified, null values sort after all non-null values; if NULLS FIRST
is specified, null values sort before all non-null values. If neither is specified, the default behavior is NULLS LAST
when ASC
is specified or implied, and NULLS FIRST
when DESC
is specified (thus, the default is to act as though nulls are larger than non-nulls). When USING
is specified, the default nulls ordering depends on whether the operator is a less-than or greater-than operator.
Note that ordering options apply only to the expression they follow; for example ORDER BY x, y DESC
does not mean the same thing as ORDER BY x DESC, y DESC
.
Character-string data is sorted according to the collation that applies to the column being sorted. That can be overridden at need by including a COLLATE
clause in the expression
, for example ORDER BY mycolumn COLLATE "en_US"
. For more information see Section 4.2.10 and Section 23.2.
LIMIT
Clause
The LIMIT
clause consists of two independent sub-clauses:
LIMIT { count | ALL } OFFSET start
count
specifies the maximum number of rows to return, while start
specifies the number of rows to skip before starting to return rows. When both are specified, start
rows are skipped before starting to count the count
rows to be returned.
If the count
expression evaluates to NULL, it is treated as LIMIT ALL
, i.e., no limit. If start
evaluates to NULL, it is treated the same as OFFSET 0
.
SQL:2008 introduced a different syntax to achieve the same result, which PostgreSQL also supports. It is:
OFFSET start { ROW | ROWS } FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY
In this syntax, to write anything except a simple integer constant for start
or count
, you must write parentheses around it. If count
is omitted in a FETCH
clause, it defaults to 1. ROW
and ROWS
as well as FIRST
and NEXT
are noise words that don't influence the effects of these clauses. According to the standard, the OFFSET
clause must come before the FETCH
clause if both are present; but PostgreSQL is laxer and allows either order.
When using LIMIT
, it is a good idea to use an ORDER BY
clause that constrains the result rows into a unique order. Otherwise you will get an unpredictable subset of the query's rows — you might be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? You don't know what ordering unless you specify ORDER BY
.
The query planner takes LIMIT
into account when generating a query plan, so you are very likely to get different plans (yielding different row orders) depending on what you use for LIMIT
and OFFSET
. Thus, using different LIMIT
/OFFSET
values to select different subsets of a query result will give inconsistent results unless you enforce a predictable result ordering with ORDER BY
. This is not a bug; it is an inherent consequence of the fact that SQL does not promise to deliver the results of a query in any particular order unless ORDER BY
is used to constrain the order.
It is even possible for repeated executions of the same LIMIT
query to return different subsets of the rows of a table, if there is not an ORDER BY
to enforce selection of a deterministic subset. Again, this is not a bug; determinism of the results is simply not guaranteed in such a case.
The Locking Clause
FOR UPDATE
, FOR NO KEY UPDATE
, FOR SHARE
and FOR KEY SHARE
are locking clauses; they affect how SELECT
locks rows as they are obtained from the table.
The locking clause has the general form
FOR lock_strength [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ]
where lock_strength
can be one of
UPDATE NO KEY UPDATE SHARE KEY SHARE
For more information on each row-level lock mode, refer to Section 13.3.2.
To prevent the operation from waiting for other transactions to commit, use either the NOWAIT
or SKIP LOCKED
option. With NOWAIT
, the statement reports an error, rather than waiting, if a selected row cannot be locked immediately. With SKIP LOCKED
, any selected rows that cannot be immediately locked are skipped. Skipping locked rows provides an inconsistent view of the data, so this is not suitable for general purpose work, but can be used to avoid lock contention with multiple consumers accessing a queue-like table. Note that NOWAIT
and SKIP LOCKED
apply only to the row-level lock(s) — the required ROW SHARE
table-level lock is still taken in the ordinary way (see Chapter 13). You can use LOCK with the NOWAIT
option first, if you need to acquire the table-level lock without waiting.
If specific tables are named in a locking clause, then only rows coming from those tables are locked; any other tables used in the SELECT
are simply read as usual. A locking clause without a table list affects all tables used in the statement. If a locking clause is applied to a view or sub-query, it affects all tables used in the view or sub-query. However, these clauses do not apply to WITH
queries referenced by the primary query. If you want row locking to occur within a WITH
query, specify a locking clause within the WITH
query.
Multiple locking clauses can be written if it is necessary to specify different locking behavior for different tables. If the same table is mentioned (or implicitly affected) by more than one locking clause, then it is processed as if it was only specified by the strongest one. Similarly, a table is processed as NOWAIT
if that is specified in any of the clauses affecting it. Otherwise, it is processed as SKIP LOCKED
if that is specified in any of the clauses affecting it.
The locking clauses cannot be used in contexts where returned rows cannot be clearly identified with individual table rows; for example they cannot be used with aggregation.
When a locking clause appears at the top level of a SELECT
query, the rows that are locked are exactly those that are returned by the query; in the case of a join query, the rows locked are those that contribute to returned join rows. In addition, rows that satisfied the query conditions as of the query snapshot will be locked, although they will not be returned if they were updated after the snapshot and no longer satisfy the query conditions. If a LIMIT
is used, locking stops once enough rows have been returned to satisfy the limit (but note that rows skipped over by OFFSET
will get locked). Similarly, if a locking clause is used in a cursor's query, only rows actually fetched or stepped past by the cursor will be locked.
When a locking clause appears in a sub-SELECT
, the rows locked are those returned to the outer query by the sub-query. This might involve fewer rows than inspection of the sub-query alone would suggest, since conditions from the outer query might be used to optimize execution of the sub-query. For example,
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
will lock only rows having col1 = 5
, even though that condition is not textually within the sub-query.
Previous releases failed to preserve a lock which is upgraded by a later savepoint. For example, this code:
BEGIN; SELECT * FROM mytable WHERE key = 1 FOR UPDATE; SAVEPOINT s; UPDATE mytable SET ... WHERE key = 1; ROLLBACK TO s;
would fail to preserve the FOR UPDATE
lock after the ROLLBACK TO
. This has been fixed in release 9.3.
Caution: It is possible for a
SELECT
command running at theREAD COMMITTED
transaction isolation level and usingORDER BY
and a locking clause to return rows out of order. This is becauseORDER BY
is applied first. The command sorts the result, but might then block trying to obtain a lock on one or more of the rows. Once theSELECT
unblocks, some of the ordering column values might have been modified, leading to those rows appearing to be out of order (though they are in order in terms of the original column values). This can be worked around at need by placing theFOR UPDATE/SHARE
clause in a sub-query, for exampleNote that this will result in locking all rows of
mytable
, whereasFOR UPDATE
at the top level would lock only the actually returned rows. This can make for a significant performance difference, particularly if theORDER BY
is combined withLIMIT
or other restrictions. So this technique is recommended only if concurrent updates of the ordering columns are expected and a strictly sorted result is required.At the
REPEATABLE READ
orSERIALIZABLE
transaction isolation level this would cause a serialization failure (with aSQLSTATE
of'40001'
), so there is no possibility of receiving rows out of order under these isolation levels.
TABLE
Command
The command
TABLE name
is equivalent to
SELECT * FROM name
It can be used as a top-level command or as a space-saving syntax variant in parts of complex queries. Only the WITH
, UNION
, INTERSECT
, EXCEPT
, ORDER BY
, LIMIT
, OFFSET
, FETCH
and FOR
locking clauses can be used with TABLE
; the WHERE
clause and any form of aggregation cannot be used.
Examples
To join the table films
with the table distributors
:
SELECT f.title, f.did, d.name, f.date_prod, f.kind FROM distributors d, films f WHERE f.did = d.did title | did | name | date_prod | kind -------------------+-----+--------------+------------+---------- The Third Man | 101 | British Lion | 1949-12-23 | Drama The African Queen | 101 | British Lion | 1951-08-11 | Romantic ...
To sum the column len
of all films and group the results by kind
:
SELECT kind, sum(len) AS total FROM films GROUP BY kind; kind | total ----------+------- Action | 07:34 Comedy | 02:58 Drama | 14:28 Musical | 06:42 Romantic | 04:38
To sum the column len
of all films, group the results by kind
and show those group totals that are less than 5 hours:
SELECT kind, sum(len) AS total FROM films GROUP BY kind HAVING sum(len) < interval '5 hours'; kind | total ----------+------- Comedy | 02:58 Romantic | 04:38
The following two examples are identical ways of sorting the individual results according to the contents of the second column (name
):
SELECT * FROM distributors ORDER BY name; SELECT * FROM distributors ORDER BY 2; did | name -----+------------------ 109 | 20th Century Fox 110 | Bavaria Atelier 101 | British Lion 107 | Columbia 102 | Jean Luc Godard 113 | Luso films 104 | Mosfilm 103 | Paramount 106 | Toho 105 | United Artists 111 | Walt Disney 112 | Warner Bros. 108 | Westward
The next example shows how to obtain the union of the tables distributors
and actors
, restricting the results to those that begin with the letter W in each table. Only distinct rows are wanted, so the key word ALL
is omitted.
distributors: actors: did | name id | name -----+-------------- ----+---------------- 108 | Westward 1 | Woody Allen 111 | Walt Disney 2 | Warren Beatty 112 | Warner Bros. 3 | Walter Matthau ... ... SELECT distributors.name FROM distributors WHERE distributors.name LIKE 'W%' UNION SELECT actors.name FROM actors WHERE actors.name LIKE 'W%'; name ---------------- Walt Disney Walter Matthau Warner Bros. Warren Beatty Westward Woody Allen
This example shows how to use a function in the FROM
clause, both with and without a column definition list:
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$ SELECT * FROM distributors WHERE did = $1; $$ LANGUAGE SQL; SELECT * FROM distributors(111); did | name -----+------------- 111 | Walt Disney CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$ SELECT * FROM distributors WHERE did = $1; $$ LANGUAGE SQL; SELECT * FROM distributors_2(111) AS (f1 int, f2 text); f1 | f2 -----+------------- 111 | Walt Disney
Here is an example of a function with an ordinality column added:
SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY; unnest | ordinality --------+---------- a | 1 b | 2 c | 3 d | 4 e | 5 f | 6 (6 rows)
This example shows how to use a simple WITH
clause:
WITH t AS ( SELECT random() as x FROM generate_series(1, 3) ) SELECT * FROM t UNION ALL SELECT * FROM t x -------------------- 0.534150459803641 0.520092216785997 0.0735620250925422 0.534150459803641 0.520092216785997 0.0735620250925422
Notice that the WITH
query was evaluated only once, so that we got two sets of the same three random values.
This example uses WITH RECURSIVE
to find all subordinates (direct or indirect) of the employee Mary, and their level of indirectness, from a table that shows only direct subordinates:
WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS ( SELECT 1, employee_name, manager_name FROM employee WHERE manager_name = 'Mary' UNION ALL SELECT er.distance + 1, e.employee_name, e.manager_name FROM employee_recursive er, employee e WHERE er.employee_name = e.manager_name ) SELECT distance, employee_name FROM employee_recursive;
Notice the typical form of recursive queries: an initial condition, followed by UNION
, followed by the recursive part of the query. Be sure that the recursive part of the query will eventually return no tuples, or else the query will loop indefinitely. (See Section 7.8 for more examples.)
This example uses LATERAL
to apply a set-returning function get_product_names()
for each row of the manufacturers
table:
SELECT m.name AS mname, pname FROM manufacturers m, LATERAL get_product_names(m.id) pname;
Manufacturers not currently having any products would not appear in the result, since it is an inner join. If we wished to include the names of such manufacturers in the result, we could do:
SELECT m.name AS mname, pname FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;
Compatibility
Of course, the SELECT
statement is compatible with the SQL standard. But there are some extensions and some missing features.
Omitted FROM
Clauses
PostgreSQL allows one to omit the FROM
clause. It has a straightforward use to compute the results of simple expressions:
SELECT 2+2; ?column? ---------- 4
Some other SQL databases cannot do this except by introducing a dummy one-row table from which to do the SELECT
.
Note that if a FROM
clause is not specified, the query cannot reference any database tables. For example, the following query is invalid:
SELECT distributors.* WHERE distributors.name = 'Westward';
PostgreSQL releases prior to 8.1 would accept queries of this form, and add an implicit entry to the query's FROM
clause for each table referenced by the query. This is no longer allowed.
Empty SELECT
Lists
The list of output expressions after SELECT
can be empty, producing a zero-column result table. This is not valid syntax according to the SQL standard. PostgreSQL allows it to be consistent with allowing zero-column tables. However, an empty list is not allowed when DISTINCT
is used.
Omitting the AS
Key Word
In the SQL standard, the optional key word AS
can be omitted before an output column name whenever the new column name is a valid column name (that is, not the same as any reserved keyword). PostgreSQL is slightly more restrictive: AS
is required if the new column name matches any keyword at all, reserved or not. Recommended practice is to use AS
or double-quote output column names, to prevent any possible conflict against future keyword additions.
In FROM
items, both the standard and PostgreSQL allow AS
to be omitted before an alias that is an unreserved keyword. But this is impractical for output column names, because of syntactic ambiguities.
ONLY
and Inheritance
The SQL standard requires parentheses around the table name when writing ONLY
, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE ...
. PostgreSQL considers these parentheses to be optional.
PostgreSQL allows a trailing *
to be written to explicitly specify the non-ONLY
behavior of including child tables. The standard does not allow this.
(These points apply equally to all SQL commands supporting the ONLY
option.)
TABLESAMPLE
Clause Restrictions
The TABLESAMPLE
clause is currently accepted only on regular tables and materialized views. According to the SQL standard it should be possible to apply it to any FROM
item.
Function Calls in FROM
PostgreSQL allows a function call to be written directly as a member of the FROM
list. In the SQL standard it would be necessary to wrap such a function call in a sub-SELECT
; that is, the syntax FROM
is approximately equivalent to func
(...) alias
FROM LATERAL (SELECT
. Note that func
(...)) alias
LATERAL
is considered to be implicit; this is because the standard requires LATERAL
semantics for an UNNEST()
item in FROM
. PostgreSQL treats UNNEST()
the same as other set-returning functions.
Namespace Available to GROUP BY
and ORDER BY
In the SQL-92 standard, an ORDER BY
clause can only use output column names or numbers, while a GROUP BY
clause can only use expressions based on input column names. PostgreSQL extends each of these clauses to allow the other choice as well (but it uses the standard's interpretation if there is ambiguity). PostgreSQL also allows both clauses to specify arbitrary expressions. Note that names appearing in an expression will always be taken as input-column names, not as output-column names.
SQL:1999 and later use a slightly different definition which is not entirely upward compatible with SQL-92. In most cases, however, PostgreSQL will interpret an ORDER BY
or GROUP BY
expression the same way SQL:1999 does.
Functional Dependencies
PostgreSQL recognizes functional dependency (allowing columns to be omitted from GROUP BY
) only when a table's primary key is included in the GROUP BY
list. The SQL standard specifies additional conditions that should be recognized.
WINDOW
Clause Restrictions
The SQL standard provides additional options for the window frame_clause
. PostgreSQL currently supports only the options listed above.
LIMIT
and OFFSET
The clauses LIMIT
and OFFSET
are PostgreSQL-specific syntax, also used by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ... FETCH {FIRST|NEXT} ...
for the same functionality, as shown above in LIMIT Clause. This syntax is also used by IBM DB2. (Applications written for Oracle frequently use a workaround involving the automatically generated rownum
column, which is not available in PostgreSQL, to implement the effects of these clauses.)
FOR NO KEY UPDATE
, FOR UPDATE
, FOR SHARE
, FOR KEY SHARE
Although FOR UPDATE
appears in the SQL standard, the standard allows it only as an option of DECLARE CURSOR
. PostgreSQL allows it in any SELECT
query as well as in sub-SELECT
s, but this is an extension. The FOR NO KEY UPDATE
, FOR SHARE
and FOR KEY SHARE
variants, as well as the NOWAIT
and SKIP LOCKED
options, do not appear in the standard.
Data-Modifying Statements in WITH
PostgreSQL allows INSERT
, UPDATE
, and DELETE
to be used as WITH
queries. This is not found in the SQL standard.
Nonstandard Clauses
DISTINCT ON ( ... )
is an extension of the SQL standard.
ROWS FROM( ... )
is an extension of the SQL standard.
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