Current File : //opt/alt/postgresql11/usr/share/man/man7/SELECT.7
'\" t
.\" Title: SELECT
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\" Date: 2017-11-06
.\" Manual: PostgreSQL 9.2.24 Documentation
.\" Source: PostgreSQL 9.2.24
.\" Language: English
.\"
.TH "SELECT" "7" "2017-11-06" "PostgreSQL 9.2.24" "PostgreSQL 9.2.24 Documentation"
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.SH "NAME"
SELECT, TABLE, WITH \- retrieve rows from a table or view
.\" SELECT
.\" TABLE command
.\" WITH: in SELECT
.SH "SYNOPSIS"
.sp
.nf
[ WITH [ RECURSIVE ] \fIwith_query\fR [, \&.\&.\&.] ]
SELECT [ ALL | DISTINCT [ ON ( \fIexpression\fR [, \&.\&.\&.] ) ] ]
* | \fIexpression\fR [ [ AS ] \fIoutput_name\fR ] [, \&.\&.\&.]
[ FROM \fIfrom_item\fR [, \&.\&.\&.] ]
[ WHERE \fIcondition\fR ]
[ GROUP BY \fIexpression\fR [, \&.\&.\&.] ]
[ HAVING \fIcondition\fR [, \&.\&.\&.] ]
[ WINDOW \fIwindow_name\fR AS ( \fIwindow_definition\fR ) [, \&.\&.\&.] ]
[ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] \fIselect\fR ]
[ ORDER BY \fIexpression\fR [ ASC | DESC | USING \fIoperator\fR ] [ NULLS { FIRST | LAST } ] [, \&.\&.\&.] ]
[ LIMIT { \fIcount\fR | ALL } ]
[ OFFSET \fIstart\fR [ ROW | ROWS ] ]
[ FETCH { FIRST | NEXT } [ \fIcount\fR ] { ROW | ROWS } ONLY ]
[ FOR { UPDATE | SHARE } [ OF \fItable_name\fR [, \&.\&.\&.] ] [ NOWAIT ] [\&.\&.\&.] ]
where \fIfrom_item\fR can be one of:
[ ONLY ] \fItable_name\fR [ * ] [ [ AS ] \fIalias\fR [ ( \fIcolumn_alias\fR [, \&.\&.\&.] ) ] ]
( \fIselect\fR ) [ AS ] \fIalias\fR [ ( \fIcolumn_alias\fR [, \&.\&.\&.] ) ]
\fIwith_query_name\fR [ [ AS ] \fIalias\fR [ ( \fIcolumn_alias\fR [, \&.\&.\&.] ) ] ]
\fIfunction_name\fR ( [ \fIargument\fR [, \&.\&.\&.] ] ) [ AS ] \fIalias\fR [ ( \fIcolumn_alias\fR [, \&.\&.\&.] | \fIcolumn_definition\fR [, \&.\&.\&.] ) ]
\fIfunction_name\fR ( [ \fIargument\fR [, \&.\&.\&.] ] ) AS ( \fIcolumn_definition\fR [, \&.\&.\&.] )
\fIfrom_item\fR [ NATURAL ] \fIjoin_type\fR \fIfrom_item\fR [ ON \fIjoin_condition\fR | USING ( \fIjoin_column\fR [, \&.\&.\&.] ) ]
and \fIwith_query\fR is:
\fIwith_query_name\fR [ ( \fIcolumn_name\fR [, \&.\&.\&.] ) ] AS ( \fIselect\fR | \fIvalues\fR | \fIinsert\fR | \fIupdate\fR | \fIdelete\fR )
TABLE [ ONLY ] \fItable_name\fR [ * ]
.fi
.SH "DESCRIPTION"
.PP
\fBSELECT\fR
retrieves rows from zero or more tables\&. The general processing of
\fBSELECT\fR
is as follows:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 1." 4.2
.\}
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\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 2." 4.2
.\}
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\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 3." 4.2
.\}
If the
WHERE
clause is specified, all rows that do not satisfy the condition are eliminated from the output\&. (See
WHERE Clause
below\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 4." 4.2
.\}
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\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 5.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 5." 4.2
.\}
The actual output rows are computed using the
\fBSELECT\fR
output expressions for each selected row or row group\&. (See
SELECT List
below\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 6.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 6." 4.2
.\}
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\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 7.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 7." 4.2
.\}
Using the operators
UNION,
INTERSECT, and
EXCEPT, the output of more than one
\fBSELECT\fR
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
\fBSELECT\fR
itself\&. (See
UNION Clause,
INTERSECT Clause, and
EXCEPT Clause
below\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 8.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 8." 4.2
.\}
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\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 9.\h'+01'\c
.\}
.el \{\
.sp -1
.IP " 9." 4.2
.\}
If the
LIMIT
(or
FETCH FIRST) or
OFFSET
clause is specified, the
\fBSELECT\fR
statement only returns a subset of the result rows\&. (See
LIMIT Clause
below\&.)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'10.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "10." 4.2
.\}
If
FOR UPDATE
or
FOR SHARE
is specified, the
\fBSELECT\fR
statement locks the selected rows against concurrent updates\&. (See
FOR UPDATE/FOR SHARE Clause
below\&.)
.RE
.PP
You must have
SELECT
privilege on each column used in a
\fBSELECT\fR
command\&. The use of
FOR UPDATE
or
FOR SHARE
requires
UPDATE
privilege as well (for at least one column of each table so selected)\&.
.SH "PARAMETERS"
.SS "WITH Clause"
.PP
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
\fBSELECT\fR,
\fBVALUES\fR,
\fBINSERT\fR,
\fBUPDATE\fR
or
\fBDELETE\fR
statement\&. When writing a data\-modifying statement (\fBINSERT\fR,
\fBUPDATE\fR
or
\fBDELETE\fR) in
WITH, it is usual to include a
RETURNING
clause\&. It is the output of
RETURNING,
\fInot\fR
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\&.
.PP
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\&.
.PP
If
RECURSIVE
is specified, it allows a
\fBSELECT\fR
subquery to reference itself by name\&. Such a subquery must have the form
.sp
.if n \{\
.RS 4
.\}
.nf
\fInon_recursive_term\fR UNION [ ALL | DISTINCT ] \fIrecursive_term\fR
.fi
.if n \{\
.RE
.\}
.sp
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
\fBSELECT\fR
query in a data\-modifying statement\&. See
Section 7.8, \(lqWITH Queries (Common Table Expressions)\(rq, in the documentation
for an example\&.
.PP
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\&.
.PP
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\&.
.PP
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\&.
.PP
See
Section 7.8, \(lqWITH Queries (Common Table Expressions)\(rq, in the documentation
for additional information\&.
.SS "FROM Clause"
.PP
The
FROM
clause specifies one or more source tables for the
\fBSELECT\fR\&. 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\&.
.PP
The
FROM
clause can contain the following elements:
.PP
\fItable_name\fR
.RS 4
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\&.
.RE
.PP
\fIalias\fR
.RS 4
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
\fBSELECT\fR
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\&.
.RE
.PP
\fIselect\fR
.RS 4
A sub\-\fBSELECT\fR
can appear in the
FROM
clause\&. This acts as though its output were created as a temporary table for the duration of this single
\fBSELECT\fR
command\&. Note that the sub\-\fBSELECT\fR
must be surrounded by parentheses, and an alias
\fImust\fR
be provided for it\&. A
\fBVALUES\fR(7)
command can also be used here\&.
.RE
.PP
\fIwith_query_name\fR
.RS 4
A
WITH
query is referenced by writing its name, just as though the query\*(Aqs 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\*(Aqs name\&.) An alias can be provided in the same way as for a table\&.
.RE
.PP
\fIfunction_name\fR
.RS 4
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
\fBSELECT\fR
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\*(Aqs 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
( \fIcolumn_name\fR \fIdata_type\fR [, \&.\&.\&. ] )\&. The column definition list must match the actual number and types of columns returned by the function\&.
.RE
.PP
\fIjoin_type\fR
.RS 4
One of
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
[ INNER ] JOIN
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LEFT [ OUTER ] JOIN
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
RIGHT [ OUTER ] JOIN
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
FULL [ OUTER ] JOIN
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
CROSS JOIN
.RE
.sp
For the
INNER
and
OUTER
join types, a join condition must be specified, namely exactly one of
NATURAL,
ON \fIjoin_condition\fR, or
USING (\fIjoin_column\fR [, \&.\&.\&.])\&. See below for the meaning\&. For
CROSS JOIN, none of these clauses can appear\&.
.sp
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\&.
.sp
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\*(Aqt do with plain
FROM
and
WHERE\&.
.sp
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\*(Aqs own condition is considered while deciding which rows have matches\&. Outer conditions are applied afterwards\&.
.sp
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\&.
.sp
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)\&.
.RE
.PP
ON \fIjoin_condition\fR
.RS 4
\fIjoin_condition\fR
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\&.
.RE
.PP
USING ( \fIjoin_column\fR [, \&.\&.\&.] )
.RS 4
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\&.
.RE
.PP
NATURAL
.RS 4
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\&.
.RE
.SS "WHERE Clause"
.PP
The optional
WHERE
clause has the general form
.sp
.if n \{\
.RS 4
.\}
.nf
WHERE \fIcondition\fR
.fi
.if n \{\
.RE
.\}
.sp
where
\fIcondition\fR
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\&.
.SS "GROUP BY Clause"
.PP
The optional
GROUP BY
clause has the general form
.sp
.if n \{\
.RS 4
.\}
.nf
GROUP BY \fIexpression\fR [, \&.\&.\&.]
.fi
.if n \{\
.RE
.\}
.PP
GROUP BY
will condense into a single row all selected rows that share the same values for the grouped expressions\&.
\fIexpression\fR
can be an input column name, or the name or ordinal number of an output column (\fBSELECT\fR
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\&.
.PP
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
\fBSELECT\fR
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\&.
.SS "HAVING Clause"
.PP
The optional
HAVING
clause has the general form
.sp
.if n \{\
.RS 4
.\}
.nf
HAVING \fIcondition\fR
.fi
.if n \{\
.RE
.\}
.sp
where
\fIcondition\fR
is the same as specified for the
WHERE
clause\&.
.PP
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
\fIcondition\fR
must unambiguously reference a grouping column, unless the reference appears within an aggregate function\&.
.PP
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
\fBSELECT\fR
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\&.
.SS "WINDOW Clause"
.PP
The optional
WINDOW
clause has the general form
.sp
.if n \{\
.RS 4
.\}
.nf
WINDOW \fIwindow_name\fR AS ( \fIwindow_definition\fR ) [, \&.\&.\&.]
.fi
.if n \{\
.RE
.\}
.sp
where
\fIwindow_name\fR
is a name that can be referenced from
OVER
clauses or subsequent window definitions, and
\fIwindow_definition\fR
is
.sp
.if n \{\
.RS 4
.\}
.nf
[ \fIexisting_window_name\fR ]
[ PARTITION BY \fIexpression\fR [, \&.\&.\&.] ]
[ ORDER BY \fIexpression\fR [ ASC | DESC | USING \fIoperator\fR ] [ NULLS { FIRST | LAST } ] [, \&.\&.\&.] ]
[ \fIframe_clause\fR ]
.fi
.if n \{\
.RE
.\}
.PP
If an
\fIexisting_window_name\fR
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\&.
.PP
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\&.
.PP
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\&.
.PP
The optional
\fIframe_clause\fR
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
\fIframe_clause\fR
can be one of
.sp
.if n \{\
.RS 4
.\}
.nf
{ RANGE | ROWS } \fIframe_start\fR
{ RANGE | ROWS } BETWEEN \fIframe_start\fR AND \fIframe_end\fR
.fi
.if n \{\
.RE
.\}
.sp
where
\fIframe_start\fR
and
\fIframe_end\fR
can be one of
.sp
.if n \{\
.RS 4
.\}
.nf
UNBOUNDED PRECEDING
\fIvalue\fR PRECEDING
CURRENT ROW
\fIvalue\fR FOLLOWING
UNBOUNDED FOLLOWING
.fi
.if n \{\
.RE
.\}
.sp
If
\fIframe_end\fR
is omitted it defaults to
CURRENT ROW\&. Restrictions are that
\fIframe_start\fR
cannot be
UNBOUNDED FOLLOWING,
\fIframe_end\fR
cannot be
UNBOUNDED PRECEDING, and the
\fIframe_end\fR
choice cannot appear earlier in the above list than the
\fIframe_start\fR
choice \(em for example
RANGE BETWEEN CURRENT ROW AND \fIvalue\fR PRECEDING
is not allowed\&.
.PP
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\*(Aqs 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\*(Aqs first or last peer in the
ORDER BY
ordering\&. The
\fIvalue\fRPRECEDING
and
\fIvalue\fRFOLLOWING
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\&.
\fIvalue\fR
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\&.
.PP
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\&.
.PP
The purpose of a
WINDOW
clause is to specify the behavior of
window functions
appearing in the query\*(Aqs
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\&.
.PP
Window functions are described in detail in
Section 3.5, \(lqWindow Functions\(rq, in the documentation,
Section 4.2.8, \(lqWindow Function Calls\(rq, in the documentation, and
Section 7.2.4, \(lqWindow Function Processing\(rq, in the documentation\&.
.SS "SELECT List"
.PP
The
\fBSELECT\fR
list (between the key words
SELECT
and
FROM) specifies expressions that form the output rows of the
\fBSELECT\fR
statement\&. The expressions can (and usually do) refer to columns computed in the
FROM
clause\&.
.PP
Just as in a table, every output column of a
\fBSELECT\fR
has a name\&. In a simple
\fBSELECT\fR
this name is just used to label the column for display, but when the
\fBSELECT\fR
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\fIoutput_name\fR
after the column\*(Aqs 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\*(Aqs expression is a simple column reference then the chosen name is the same as that column\*(Aqs 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?\&.
.PP
An output column\*(Aqs name can be used to refer to the column\*(Aqs value in
ORDER BY
and
GROUP BY
clauses, but not in the
WHERE
or
HAVING
clauses; there you must write out the expression instead\&.
.PP
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
\fItable_name\fR\&.*
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\*(Aq names\&.
.SS "DISTINCT Clause"
.PP
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\&.
.PP
SELECT DISTINCT ON ( \fIexpression\fR [, \&.\&.\&.] )
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
\(lqfirst row\(rq
of each set is unpredictable unless
ORDER BY
is used to ensure that the desired row appears first\&. For example:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT DISTINCT ON (location) location, time, report
FROM weather_reports
ORDER BY location, time DESC;
.fi
.if n \{\
.RE
.\}
.sp
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\*(Aqd have gotten a report from an unpredictable time for each location\&.
.PP
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\&.
.SS "UNION Clause"
.PP
The
UNION
clause has this general form:
.sp
.if n \{\
.RS 4
.\}
.nf
\fIselect_statement\fR UNION [ ALL | DISTINCT ] \fIselect_statement\fR
.fi
.if n \{\
.RE
.\}
.sp
\fIselect_statement\fR
is any
\fBSELECT\fR
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\&.)
.PP
The
UNION
operator computes the set union of the rows returned by the involved
\fBSELECT\fR
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
\fBSELECT\fR
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\&.
.PP
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\&.
.PP
Multiple
UNION
operators in the same
\fBSELECT\fR
statement are evaluated left to right, unless otherwise indicated by parentheses\&.
.PP
Currently,
FOR UPDATE
and
FOR SHARE
cannot be specified either for a
UNION
result or for any input of a
UNION\&.
.SS "INTERSECT Clause"
.PP
The
INTERSECT
clause has this general form:
.sp
.if n \{\
.RS 4
.\}
.nf
\fIselect_statement\fR INTERSECT [ ALL | DISTINCT ] \fIselect_statement\fR
.fi
.if n \{\
.RE
.\}
.sp
\fIselect_statement\fR
is any
\fBSELECT\fR
statement without an
ORDER BY,
LIMIT,
FOR UPDATE, or
FOR SHARE
clause\&.
.PP
The
INTERSECT
operator computes the set intersection of the rows returned by the involved
\fBSELECT\fR
statements\&. A row is in the intersection of two result sets if it appears in both result sets\&.
.PP
The result of
INTERSECT
does not contain any duplicate rows unless the
ALL
option is specified\&. With
ALL, a row that has
\fIm\fR
duplicates in the left table and
\fIn\fR
duplicates in the right table will appear min(\fIm\fR,\fIn\fR) times in the result set\&.
DISTINCT
can be written to explicitly specify the default behavior of eliminating duplicate rows\&.
.PP
Multiple
INTERSECT
operators in the same
\fBSELECT\fR
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)\&.
.PP
Currently,
FOR UPDATE
and
FOR SHARE
cannot be specified either for an
INTERSECT
result or for any input of an
INTERSECT\&.
.SS "EXCEPT Clause"
.PP
The
EXCEPT
clause has this general form:
.sp
.if n \{\
.RS 4
.\}
.nf
\fIselect_statement\fR EXCEPT [ ALL | DISTINCT ] \fIselect_statement\fR
.fi
.if n \{\
.RE
.\}
.sp
\fIselect_statement\fR
is any
\fBSELECT\fR
statement without an
ORDER BY,
LIMIT,
FOR UPDATE, or
FOR SHARE
clause\&.
.PP
The
EXCEPT
operator computes the set of rows that are in the result of the left
\fBSELECT\fR
statement but not in the result of the right one\&.
.PP
The result of
EXCEPT
does not contain any duplicate rows unless the
ALL
option is specified\&. With
ALL, a row that has
\fIm\fR
duplicates in the left table and
\fIn\fR
duplicates in the right table will appear max(\fIm\fR\-\fIn\fR,0) times in the result set\&.
DISTINCT
can be written to explicitly specify the default behavior of eliminating duplicate rows\&.
.PP
Multiple
EXCEPT
operators in the same
\fBSELECT\fR
statement are evaluated left to right, unless parentheses dictate otherwise\&.
EXCEPT
binds at the same level as
UNION\&.
.PP
Currently,
FOR UPDATE
and
FOR SHARE
cannot be specified either for an
EXCEPT
result or for any input of an
EXCEPT\&.
.SS "ORDER BY Clause"
.PP
The optional
ORDER BY
clause has this general form:
.sp
.if n \{\
.RS 4
.\}
.nf
ORDER BY \fIexpression\fR [ ASC | DESC | USING \fIoperator\fR ] [ NULLS { FIRST | LAST } ] [, \&.\&.\&.]
.fi
.if n \{\
.RE
.\}
.sp
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\&.
.PP
Each
\fIexpression\fR
can be the name or ordinal number of an output column (\fBSELECT\fR
list item), or it can be an arbitrary expression formed from input\-column values\&.
.PP
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\&.
.PP
It is also possible to use arbitrary expressions in the
ORDER BY
clause, including columns that do not appear in the
\fBSELECT\fR
output list\&. Thus the following statement is valid:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT name FROM distributors ORDER BY code;
.fi
.if n \{\
.RE
.\}
.sp
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\&.
.PP
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\&.
.PP
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\&.)
.PP
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\&.
.PP
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\&.
.PP
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
\fIexpression\fR, for example
ORDER BY mycolumn COLLATE "en_US"\&. For more information see
Section 4.2.10, \(lqCollation Expressions\(rq, in the documentation
and
Section 22.2, \(lqCollation Support\(rq, in the documentation\&.
.SS "LIMIT Clause"
.PP
The
LIMIT
clause consists of two independent sub\-clauses:
.sp
.if n \{\
.RS 4
.\}
.nf
LIMIT { \fIcount\fR | ALL }
OFFSET \fIstart\fR
.fi
.if n \{\
.RE
.\}
.sp
\fIcount\fR
specifies the maximum number of rows to return, while
\fIstart\fR
specifies the number of rows to skip before starting to return rows\&. When both are specified,
\fIstart\fR
rows are skipped before starting to count the
\fIcount\fR
rows to be returned\&.
.PP
If the
\fIcount\fR
expression evaluates to NULL, it is treated as
LIMIT ALL, i\&.e\&., no limit\&. If
\fIstart\fR
evaluates to NULL, it is treated the same as
OFFSET 0\&.
.PP
SQL:2008 introduced a different syntax to achieve the same result, which
PostgreSQL
also supports\&. It is:
.sp
.if n \{\
.RS 4
.\}
.nf
OFFSET \fIstart\fR { ROW | ROWS }
FETCH { FIRST | NEXT } [ \fIcount\fR ] { ROW | ROWS } ONLY
.fi
.if n \{\
.RE
.\}
.sp
In this syntax, to write anything except a simple integer constant for
\fIstart\fR
or
\fIcount\fR, you must write parentheses around it\&. If
\fIcount\fR
is omitted in a
FETCH
clause, it defaults to 1\&.
ROW
and
ROWS
as well as
FIRST
and
NEXT
are noise words that don\*(Aqt 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\&.
.PP
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\*(Aqs rows \(em you might be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? You don\*(Aqt know what ordering unless you specify
ORDER BY\&.
.PP
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
\fIwill give inconsistent results\fR
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\&.
.PP
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\&.
.SS "FOR UPDATE/FOR SHARE Clause"
.PP
The
FOR UPDATE
clause has this form:
.sp
.if n \{\
.RS 4
.\}
.nf
FOR UPDATE [ OF \fItable_name\fR [, \&.\&.\&.] ] [ NOWAIT ]
.fi
.if n \{\
.RE
.\}
.PP
The closely related
FOR SHARE
clause has this form:
.sp
.if n \{\
.RS 4
.\}
.nf
FOR SHARE [ OF \fItable_name\fR [, \&.\&.\&.] ] [ NOWAIT ]
.fi
.if n \{\
.RE
.\}
.PP
FOR UPDATE
causes the rows retrieved by the
\fBSELECT\fR
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
\fBUPDATE\fR,
\fBDELETE\fR, or
\fBSELECT FOR UPDATE\fR
of these rows will be blocked until the current transaction ends\&. Also, if an
\fBUPDATE\fR,
\fBDELETE\fR, or
\fBSELECT FOR UPDATE\fR
from another transaction has already locked a selected row or rows,
\fBSELECT FOR UPDATE\fR
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\&.
.PP
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
\fBUPDATE\fR,
\fBDELETE\fR, or
\fBSELECT FOR UPDATE\fR
on these rows, but it does not prevent them from performing
\fBSELECT FOR SHARE\fR\&.
.PP
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) \(em 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
\fBLOCK\fR(7)
with the
NOWAIT
option first, if you need to acquire the table\-level lock without waiting\&.
.PP
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
\fBSELECT\fR
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\&.
.PP
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\&.
.PP
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\&.
.PP
When
FOR UPDATE
or
FOR SHARE
appears at the top level of a
\fBSELECT\fR
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\*(Aqs query, only rows actually fetched or stepped past by the cursor will be locked\&.
.PP
When
FOR UPDATE
or
FOR SHARE
appears in a sub\-\fBSELECT\fR, 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,
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
.fi
.if n \{\
.RE
.\}
.sp
will lock only rows having
col1 = 5, even though that condition is not textually within the sub\-query\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBCaution\fR
.ps -1
.br
.PP
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:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN;
SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
SAVEPOINT s;
UPDATE mytable SET \&.\&.\&. WHERE key = 1;
ROLLBACK TO s;
.fi
.if n \{\
.RE
.\}
.sp
After the
\fBROLLBACK\fR, 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\&.
.sp .5v
.RE
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBCaution\fR
.ps -1
.br
.PP
It is possible for a
\fBSELECT\fR
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
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
.fi
.if n \{\
.RE
.\}
.sp
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\&.
.PP
At the
REPEATABLE READ
or
SERIALIZABLE
transaction isolation level this would cause a serialization failure (with a
SQLSTATE
of
\*(Aq40001\*(Aq), so there is no possibility of receiving rows out of order under these isolation levels\&.
.sp .5v
.RE
.SS "TABLE Command"
.PP
The command
.sp
.if n \{\
.RS 4
.\}
.nf
TABLE \fIname\fR
.fi
.if n \{\
.RE
.\}
.sp
is completely equivalent to
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT * FROM \fIname\fR
.fi
.if n \{\
.RE
.\}
.sp
It can be used as a top\-level command or as a space\-saving syntax variant in parts of complex queries\&.
.SH "EXAMPLES"
.PP
To join the table
films
with the table
distributors:
.sp
.if n \{\
.RS 4
.\}
.nf
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
\&.\&.\&.
.fi
.if n \{\
.RE
.\}
.PP
To sum the column
len
of all films and group the results by
kind:
.sp
.if n \{\
.RS 4
.\}
.nf
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
.fi
.if n \{\
.RE
.\}
.PP
To sum the column
len
of all films, group the results by
kind
and show those group totals that are less than 5 hours:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT kind, sum(len) AS total
FROM films
GROUP BY kind
HAVING sum(len) < interval \*(Aq5 hours\*(Aq;
kind | total
\-\-\-\-\-\-\-\-\-\-+\-\-\-\-\-\-\-
Comedy | 02:58
Romantic | 04:38
.fi
.if n \{\
.RE
.\}
.PP
The following two examples are identical ways of sorting the individual results according to the contents of the second column (name):
.sp
.if n \{\
.RS 4
.\}
.nf
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
.fi
.if n \{\
.RE
.\}
.PP
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\&.
.sp
.if n \{\
.RS 4
.\}
.nf
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 \*(AqW%\*(Aq
UNION
SELECT actors\&.name
FROM actors
WHERE actors\&.name LIKE \*(AqW%\*(Aq;
name
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-
Walt Disney
Walter Matthau
Warner Bros\&.
Warren Beatty
Westward
Woody Allen
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.PP
This example shows how to use a function in the
FROM
clause, both with and without a column definition list:
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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
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.PP
This example shows how to use a simple
WITH
clause:
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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
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Notice that the
WITH
query was evaluated only once, so that we got two sets of the same three random values\&.
.PP
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:
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WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
SELECT 1, employee_name, manager_name
FROM employee
WHERE manager_name = \*(AqMary\*(Aq
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;
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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, \(lqWITH Queries (Common Table Expressions)\(rq, in the documentation
for more examples\&.)
.SH "COMPATIBILITY"
.PP
Of course, the
\fBSELECT\fR
statement is compatible with the SQL standard\&. But there are some extensions and some missing features\&.
.SS "Omitted FROM Clauses"
.PP
PostgreSQL
allows one to omit the
FROM
clause\&. It has a straightforward use to compute the results of simple expressions:
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SELECT 2+2;
?column?
\-\-\-\-\-\-\-\-\-\-
4
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Some other
SQL
databases cannot do this except by introducing a dummy one\-row table from which to do the
\fBSELECT\fR\&.
.PP
Note that if a
FROM
clause is not specified, the query cannot reference any database tables\&. For example, the following query is invalid:
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SELECT distributors\&.* WHERE distributors\&.name = \*(AqWestward\*(Aq;
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PostgreSQL
releases prior to 8\&.1 would accept queries of this form, and add an implicit entry to the query\*(Aqs
FROM
clause for each table referenced by the query\&. This is no longer allowed\&.
.SS "Omitting the AS Key Word"
.PP
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\&.
.PP
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\&.
.SS "ONLY and Inheritance"
.PP
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\&.
.PP
PostgreSQL
allows a trailing
*
to be written to explicitly specify the non\-ONLY
behavior of including child tables\&. The standard does not allow this\&.
.PP
(These points apply equally to all SQL commands supporting the
ONLY
option\&.)
.SS "Namespace Available to GROUP BY and ORDER BY"
.PP
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\*(Aqs 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\&.
.PP
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\&.
.SS "Functional Dependencies"
.PP
PostgreSQL
recognizes functional dependency (allowing columns to be omitted from
GROUP BY) only when a table\*(Aqs primary key is included in the
GROUP BY
list\&. The SQL standard specifies additional conditions that should be recognized\&.
.SS "WINDOW Clause Restrictions"
.PP
The SQL standard provides additional options for the window
\fIframe_clause\fR\&.
PostgreSQL
currently supports only the options listed above\&.
.SS "LIMIT and OFFSET"
.PP
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\&.)
.SS "FOR UPDATE and FOR SHARE"
.PP
Although
FOR UPDATE
appears in the SQL standard, the standard allows it only as an option of
\fBDECLARE CURSOR\fR\&.
PostgreSQL
allows it in any
\fBSELECT\fR
query as well as in sub\-\fBSELECT\fRs, but this is an extension\&. The
FOR SHARE
variant, and the
NOWAIT
option, do not appear in the standard\&.
.SS "Data\-Modifying Statements in WITH"
.PP
PostgreSQL
allows
\fBINSERT\fR,
\fBUPDATE\fR, and
\fBDELETE\fR
to be used as
WITH
queries\&. This is not found in the SQL standard\&.
.SS "Nonstandard Clauses"
.PP
The clause
DISTINCT ON
is not defined in the SQL standard\&.
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