SELECT(7) PostgreSQL 9.2.7 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
· [ INNER ] JOIN
· LEFT [ OUTER ] JOIN
· RIGHT [ OUTER ] JOIN
· FULL [ OUTER ] JOIN
· 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 the same names.
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 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; 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
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, 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 9.2.7 2014-02-17 SELECT(7)
