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SELECT(7) PostgreSQL 9.2.24 Documentation SELECT(7)
NAME
SELECT, TABLE, WITH - retrieve rows from a table or view
SYNOPSIS
[ WITH [ RECURSIVE ] with_query [, ...] ]
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
* | expression [ [ AS ] output_name ] [, ...]
[ FROM from_item [, ...] ]
[ WHERE condition ]
[ GROUP BY expression [, ...] ]
[ 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 | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] [...] ]
where from_item can be one of:
[ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]
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:
1. All queries in the WITH list are computed. These effectively serve as temporary tables
that can be referenced in the FROM list. A WITH query that is referenced more than
once in FROM is computed only once. (See WITH Clause below.)
2. All elements in the FROM list are computed. (Each element in the FROM list is a real
or virtual table.) If more than one element is specified in the FROM list, they are
cross-joined together. (See FROM Clause below.)
3. If the WHERE clause is specified, all rows that do not satisfy the condition are
eliminated from the output. (See WHERE Clause below.)
4. If the GROUP BY clause is specified, the output is combined into groups of rows that
match on one or more values. If the HAVING clause is present, it eliminates groups
that do not satisfy the given condition. (See GROUP BY Clause and HAVING Clause
below.)
5. The actual output rows are computed using the SELECT output expressions for each
selected row or row group. (See SELECT List below.)
6. 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.)
7. Using the operators UNION, INTERSECT, and EXCEPT, the output of more than one SELECT
statement can be combined to form a single result set. The UNION operator returns all
rows that are in one or both of the result sets. The INTERSECT operator returns all
rows that are strictly in both result sets. The EXCEPT operator returns the rows that
are in the first result set but not in the second. In all three cases, duplicate rows
are eliminated unless ALL is specified. The noise word DISTINCT can be added to
explicitly specify eliminating duplicate rows. Notice that DISTINCT is the default
behavior here, even though ALL is the default for SELECT itself. (See UNION Clause,
INTERSECT Clause, and EXCEPT Clause below.)
8. If the ORDER BY clause is specified, the returned rows are sorted in the specified
order. If ORDER BY is not given, the rows are returned in whatever order the system
finds fastest to produce. (See ORDER BY Clause below.)
9. If the LIMIT (or FETCH FIRST) or OFFSET clause is specified, the SELECT statement only
returns a subset of the result rows. (See LIMIT Clause below.)
10. If FOR UPDATE or FOR SHARE is specified, the SELECT statement locks the selected rows
against concurrent updates. (See FOR UPDATE/FOR SHARE Clause below.)
You must have SELECT privilege on each column used in a SELECT command. The use of FOR
UPDATE or FOR 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, 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, "WITH Queries (Common Table Expressions)", in
the documentation 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, "WITH Queries (Common Table Expressions)", in the documentation 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 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. If ONLY 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 given FROM foo AS f, the remainder of the SELECT must refer to
this FROM item as f not foo. 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.
select
A sub-SELECT can appear in the FROM clause. This acts as though its output were
created as a temporary table for the duration of this single SELECT command. Note that
the sub-SELECT must be surrounded by parentheses, and an alias must be provided for
it. A VALUES(7) 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, the WITH 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 its output
were created as a temporary table for the duration of this single SELECT command. An
alias can also be used. 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. If the function has been defined as returning the record data
type, then an alias or the key word AS must be present, followed by a column
definition list in the form ( column_name data_type [, ... ] ). The column definition
list must match the actual number and types of columns returned by the function.
join_type
One of
o [ INNER ] JOIN
o LEFT [ OUTER ] JOIN
o RIGHT [ OUTER ] JOIN
o FULL [ OUTER ] JOIN
o CROSS JOIN
For the INNER and OUTER join types, a join condition must be specified, namely exactly
one of NATURAL, ON join_condition, or USING (join_column [, ...]). See below for the
meaning. For CROSS JOIN, none of these clauses can appear.
A JOIN clause combines two FROM items. Use parentheses if necessary to determine the
order of nesting. In the absence of parentheses, JOINs nest left-to-right. In any case
JOIN binds more tightly than the commas separating FROM items.
CROSS JOIN and INNER JOIN produce a simple Cartesian product, the same result as you
get from listing the two items at the top level of FROM, but restricted by the join
condition (if any). CROSS JOIN is equivalent to INNER 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 plain FROM and WHERE.
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 the JOIN 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 a LEFT OUTER JOIN by switching the left and
right inputs.
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 type boolean (similar to a
WHERE 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 for ON 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 a USING list that mentions all columns in the two tables that
have matching names. If there are no common column names, NATURAL is equivalent to ON
TRUE.
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 expression [, ...]
GROUP BY will condense into a single row all selected rows that share the same values for
the grouped expressions. expression 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.
Aggregate functions, if any are used, are computed across all rows making up each group,
producing a separate value for each group (whereas without GROUP BY, an aggregate produces
a single value computed across all the selected rows). When GROUP BY is present, it is not
valid for the SELECT list expressions to refer to ungrouped columns except within
aggregate functions or if 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.
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.
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.
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 value PRECEDING is not allowed.
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 in the ORDER BY ordering (which
means 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 valuePRECEDING and valueFOLLOWING 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; any two peer rows will be both in or
both not in the 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.
Window functions are described in detail in Section 3.5, "Window Functions", in the
documentation, Section 4.2.8, "Window Function Calls", in the documentation, and Section
7.2.4, "Window Function Processing", in the documentation.
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
ASoutput_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, SQL Key Words). 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 table_name.* as a shorthand for the
columns coming from just that table. In these cases it is not possible to specify new
names with AS; the output column names will be the same as the table columns' names.
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 ( expression [, ...] ) keeps only the first row of each set of rows
where the given expressions evaluate to equal. The 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.
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 UPDATE, or FOR
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 UPDATE and FOR 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 UPDATE, or FOR
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 UPDATE and FOR 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 UPDATE, or FOR
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 UPDATE and FOR 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, "Collation Expressions", in the documentation and Section 22.2, "Collation
Support", in the documentation.
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.
FOR UPDATE/FOR SHARE Clause
The FOR UPDATE clause has this form:
FOR UPDATE [ OF table_name [, ...] ] [ NOWAIT ]
The closely related FOR SHARE clause has this form:
FOR SHARE [ OF table_name [, ...] ] [ NOWAIT ]
FOR UPDATE causes the rows retrieved by the SELECT statement to be locked as though for
update. This prevents them from being modified or deleted by other transactions until the
current transaction ends. That is, other transactions that attempt UPDATE, DELETE, or
SELECT FOR UPDATE of these rows will be blocked until the current transaction ends. Also,
if an UPDATE, DELETE, or SELECT FOR UPDATE from another transaction has already locked a
selected row or rows, SELECT FOR UPDATE will wait for the other transaction to complete,
and will then lock and return the updated row (or no row, if the row was deleted). Within
a REPEATABLE READ or SERIALIZABLE transaction, however, an error will be thrown if a row
to be locked has changed since the transaction started. For further discussion see Chapter
13, Concurrency Control, in the documentation.
FOR SHARE behaves similarly, except that it acquires a shared rather than exclusive lock
on each retrieved row. A shared lock blocks other transactions from performing UPDATE,
DELETE, or SELECT FOR UPDATE on these rows, but it does not prevent them from performing
SELECT FOR SHARE.
To prevent the operation from waiting for other transactions to commit, use the NOWAIT
option. With NOWAIT, the statement reports an error, rather than waiting, if a selected
row cannot be locked immediately. Note that NOWAIT applies only to the row-level lock(s)
-- the required ROW SHARE table-level lock is still taken in the ordinary way (see Chapter
13, Concurrency Control, in the documentation). You can use LOCK(7) with the NOWAIT option
first, if you need to acquire the table-level lock without waiting.
If specific tables are named in FOR UPDATE or FOR SHARE, then only rows coming from those
tables are locked; any other tables used in the SELECT are simply read as usual. A FOR
UPDATE or FOR SHARE clause without a table list affects all tables used in the statement.
If FOR UPDATE or FOR SHARE is applied to a view or sub-query, it affects all tables used
in the view or sub-query. However, FOR UPDATE/FOR SHARE do not apply to WITH queries
referenced by the primary query. If you want row locking to occur within a WITH query,
specify FOR UPDATE or FOR SHARE within the WITH query.
Multiple FOR UPDATE and FOR SHARE 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 both FOR UPDATE and FOR SHARE clauses, then it is processed as FOR
UPDATE. Similarly, a table is processed as NOWAIT if that is specified in any of the
clauses affecting it.
FOR UPDATE and FOR SHARE 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 FOR UPDATE or FOR SHARE 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 FOR UPDATE or FOR SHARE is used in a cursor's query, only rows actually fetched or
stepped past by the cursor will be locked.
When FOR UPDATE or FOR SHARE 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.
Caution
Avoid locking a row and then modifying it within a later savepoint or PL/pgSQL
exception block. A subsequent rollback would cause the lock to be lost. For example:
BEGIN;
SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
SAVEPOINT s;
UPDATE mytable SET ... WHERE key = 1;
ROLLBACK TO s;
After the ROLLBACK, the row is effectively unlocked, rather than returned to its
pre-savepoint state of being locked but not modified. This hazard occurs if a row
locked in the current transaction is updated or deleted, or if a shared lock is
upgraded to exclusive: in all these cases, the former lock state is forgotten. If the
transaction is then rolled back to a state between the original locking command and
the subsequent change, the row will appear not to be locked at all. This is an
implementation deficiency which will be addressed in a future release of PostgreSQL.
Caution
It is possible for a SELECT command running at the READ COMMITTED transaction
isolation level and using ORDER BY and FOR UPDATE/SHARE to return rows out of order.
This is because ORDER 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 the SELECT
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 the FOR
UPDATE/SHARE clause in a sub-query, for example
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
Note that this will result in locking all rows of mytable, whereas FOR UPDATE at the
top level would lock only the actually returned rows. This can make for a significant
performance difference, particularly if the ORDER BY is combined with LIMIT 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 or SERIALIZABLE transaction isolation level this would cause a
serialization failure (with a SQLSTATE of '40001'), so there is no possibility of
receiving rows out of order under these isolation levels.
TABLE Command
The command
TABLE name
is completely 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.
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
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, "WITH Queries (Common Table Expressions)", in the documentation for more examples.)
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.
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.)
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 UPDATE and FOR 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-SELECTs,
but this is an extension. The FOR SHARE variant, and the NOWAIT option, 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
The clause DISTINCT ON is not defined in the SQL standard.
PostgreSQL 9.2.24 2017-11-06 SELECT(7)
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