select.7

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.\\" auto-generated by docbook2man-spec $Revision: 1.1.1.1 $
.TH "SELECT" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
SELECT \- retrieve rows from a table or view

.SH SYNOPSIS
.sp
.nf
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 [, ...] ]
    [ { UNION | INTERSECT | EXCEPT } [ ALL ] \fIselect\fR ]
    [ ORDER BY \fIexpression\fR [ ASC | DESC | USING \fIoperator\fR ] [, ...] ]
    [ LIMIT { \fIcount\fR | ALL } ]
    [ OFFSET \fIstart\fR ]
    [ 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 [, ...] ) ]
    \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 [, ...] ) ]
.sp
.fi
.SH "DESCRIPTION"
.PP
\fBSELECT\fR retrieves rows from zero or more tables.
The general processing of \fBSELECT\fR is as follows:
.IP 1. 
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 [\fBselect\fR(7)] below.)
.IP 2. 
If the WHERE clause is specified, all rows
that do not satisfy the condition are eliminated from the
output. (See WHERE Clause [\fBselect\fR(7)] below.)
.IP 3. 
If the GROUP BY clause is specified, the
output is divided 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 [\fBselect\fR(7)] and
HAVING Clause [\fBselect\fR(7)] below.)
.IP 4. 
The actual output rows are computed using the
\fBSELECT\fR output expressions for each selected
row. (See
SELECT List [\fBselect\fR(7)]
below.)
.IP 5. 
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. (See
UNION Clause [\fBselect\fR(7)], INTERSECT Clause [\fBselect\fR(l)], and
EXCEPT Clause [\fBselect\fR(7)] below.)
.IP 6. 
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 [\fBselect\fR(7)] below.)
.IP 7. 
DISTINCT eliminates duplicate rows from the
result. DISTINCT ON eliminates rows that
match on all the specified expressions. ALL
(the default) will return all candidate rows, including
duplicates. (See DISTINCT Clause [\fBselect\fR(7)] below.)
.IP 8. 
If the LIMIT or OFFSET
clause is specified, the \fBSELECT\fR statement
only returns a subset of the result rows. (See LIMIT Clause [\fBselect\fR(7)] below.)
.IP 9. 
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 [\fBselect\fR(7)] below.)
.PP
.PP
You must have SELECT privilege on a table to
read its values. The use of FOR UPDATE or
FOR SHARE requires
UPDATE privilege as well.
.SH "PARAMETERS"
.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:
.TP
\fB\fItable_name\fB\fR
The name (optionally schema-qualified) of an existing table or
view. If ONLY is specified, only that table is
scanned. If ONLY is not specified, the table and
all its descendant tables (if any) are scanned. *
can be appended to the table name to indicate that descendant
tables are to be scanned, but in the current version, this is
the default behavior. (In releases before 7.1,
ONLY was the default behavior.) The default
behavior can be modified by changing the sql_inheritance configuration option.
.TP
\fB\fIalias\fB\fR
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.
.TP
\fB\fIselect\fB\fR
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 \fBmust\fR be
provided for it. A
VALUES [\fBvalues\fR(7)] command
can also be used here.
.TP
\fB\fIfunction_name\fB\fR
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 may 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 \fBrecord\fR
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.
.TP
\fB\fIjoin_type\fB\fR
One of
.RS
.TP 0.2i
\(bu
[ INNER ] JOIN
.TP 0.2i
\(bu
LEFT [ OUTER ] JOIN
.TP 0.2i
\(bu
RIGHT [ OUTER ] JOIN
.TP 0.2i
\(bu
FULL [ OUTER ] JOIN
.TP 0.2i
\(bu
CROSS JOIN
.RE
.PP
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 may 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).
.TP
\fBON \fIjoin_condition\fB\fR
\fIjoin_condition\fR is
an expression resulting in a value of type
\fBboolean\fR (similar to a WHERE
clause) that specifies which rows in a join are considered to
match.
.TP
\fBUSING (\fIjoin_column\fB [, ...])\fR
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.
.TP
\fBNATURAL\fR
NATURAL is shorthand for a
USING list that mentions all columns in the two
tables that have the same names.
.PP
.SS "WHERE CLAUSE"
.PP
The optional WHERE clause has the general form
.sp
.nf
WHERE \fIcondition\fR
.sp
.fi
where \fIcondition\fR is
any expression that evaluates to a result of type
\fBboolean\fR. 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
.nf
GROUP BY \fIexpression\fR [, ...]
.sp
.fi
.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, since there
would be more than one possible value to return for an ungrouped
column.
.SS "HAVING CLAUSE"
.PP
The optional HAVING clause has the general form
.sp
.nf
HAVING \fIcondition\fR
.sp
.fi
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 "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. Using the clause
AS \fIoutput_name\fR, another
name can be specified for an output column. This name is
primarily used to label the column for display. It can also 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.
.PP
Instead of an expression, * can be written in
the output list as a shorthand for all the columns of the selected
rows. Also, one can write \fItable_name\fR.* as a
shorthand for the columns coming from just that table.
.SS "UNION CLAUSE"
.PP
The UNION clause has this general form:
.sp
.nf
\fIselect_statement\fR UNION [ ALL ] \fIselect_statement\fR
.sp
.fi
\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.)
.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 may not 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
.nf
\fIselect_statement\fR INTERSECT [ ALL ] \fIselect_statement\fR
.sp
.fi
\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.
.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 may not 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
.nf
\fIselect_statement\fR EXCEPT [ ALL ] \fIselect_statement\fR
.sp
.fi
\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.
.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 may not 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: