This section describes the SQL-compliant subquery expressions available in PostgreSQL. All of the expression forms documented in this section return Boolean (true/false) results.
EXISTS (subquery
)
The argument of EXISTS
is an arbitrary SELECT
statement,
or subquery. The
subquery is evaluated to determine whether it returns any rows.
If it returns at least one row, the result of EXISTS
is
“true”; if the subquery returns no rows, the result of EXISTS
is “false”.
The subquery can refer to variables from the surrounding query, which will act as constants during any one evaluation of the subquery.
The subquery will generally only be executed far enough to determine whether at least one row is returned, not all the way to completion. It is unwise to write a subquery that has any side effects (such as calling sequence functions); whether the side effects occur or not may be difficult to predict.
Since the result depends only on whether any rows are returned,
and not on the contents of those rows, the output list of the
subquery is normally uninteresting. A common coding convention is
to write all EXISTS
tests in the form
EXISTS(SELECT 1 WHERE ...)
. There are exceptions to
this rule however, such as subqueries that use INTERSECT
.
This simple example is like an inner join on col2
, but
it produces at most one output row for each tab1
row,
even if there are multiple matching tab2
rows:
SELECT col1 FROM tab1 WHERE EXISTS(SELECT 1 FROM tab2 WHERE col2 = tab1.col2);
expression
IN (subquery
)
The right-hand side is a parenthesized
subquery, which must return exactly one column. The left-hand expression
is evaluated and compared to each row of the subquery result.
The result of IN
is “true” if any equal subquery row is found.
The result is “false” if no equal row is found (including the special
case where the subquery returns no rows).
Note that if the left-hand expression yields null, or if there are
no equal right-hand values and at least one right-hand row yields
null, the result of the IN
construct will be null, not false.
This is in accordance with SQL's normal rules for Boolean combinations
of null values.
As with EXISTS
, it's unwise to assume that the subquery will
be evaluated completely.
row_constructor
IN (subquery
)
The left-hand side of this form of IN
is a row constructor,
as described in Section 4.2.11, “Row Constructors”.
The right-hand side is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result.
The result of IN
is “true” if any equal subquery row is found.
The result is “false” if no equal row is found (including the special
case where the subquery returns no rows).
As usual, null values in the rows are combined per
the normal rules of SQL Boolean expressions. Two rows are considered
equal if all their corresponding members are non-null and equal; the rows
are unequal if any corresponding members are non-null and unequal;
otherwise the result of that row comparison is unknown (null).
If all the row results are either unequal or null, with at least one null,
then the result of IN
is null.
expression
NOT IN (subquery
)
The right-hand side is a parenthesized
subquery, which must return exactly one column. The left-hand expression
is evaluated and compared to each row of the subquery result.
The result of NOT IN
is “true” if only unequal subquery rows
are found (including the special case where the subquery returns no rows).
The result is “false” if any equal row is found.
Note that if the left-hand expression yields null, or if there are
no equal right-hand values and at least one right-hand row yields
null, the result of the NOT IN
construct will be null, not true.
This is in accordance with SQL's normal rules for Boolean combinations
of null values.
As with EXISTS
, it's unwise to assume that the subquery will
be evaluated completely.
row_constructor
NOT IN (subquery
)
The left-hand side of this form of NOT IN
is a row constructor,
as described in Section 4.2.11, “Row Constructors”.
The right-hand side is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result.
The result of NOT IN
is “true” if only unequal subquery rows
are found (including the special case where the subquery returns no rows).
The result is “false” if any equal row is found.
As usual, null values in the rows are combined per
the normal rules of SQL Boolean expressions. Two rows are considered
equal if all their corresponding members are non-null and equal; the rows
are unequal if any corresponding members are non-null and unequal;
otherwise the result of that row comparison is unknown (null).
If all the row results are either unequal or null, with at least one null,
then the result of NOT IN
is null.
expression
operator
ANY (subquery
)expression
operator
SOME (subquery
)
The right-hand side is a parenthesized
subquery, which must return exactly one column. The left-hand expression
is evaluated and compared to each row of the subquery result using the
given operator
, which must yield a Boolean
result.
The result of ANY
is “true” if any true result is obtained.
The result is “false” if no true result is found (including the special
case where the subquery returns no rows).
SOME
is a synonym for ANY
.
IN
is equivalent to = ANY
.
Note that if there are no successes and at least one right-hand row yields
null for the operator's result, the result of the ANY
construct
will be null, not false.
This is in accordance with SQL's normal rules for Boolean combinations
of null values.
As with EXISTS
, it's unwise to assume that the subquery will
be evaluated completely.
row_constructor
operator
ANY (subquery
)row_constructor
operator
SOME (subquery
)
The left-hand side of this form of ANY
is a row constructor,
as described in Section 4.2.11, “Row Constructors”.
The right-hand side is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result,
using the given operator
. Presently,
only =
and <>
operators are allowed
in row-wise ANY
constructs.
The result of ANY
is “true” if any equal or unequal row is
found, respectively.
The result is “false” if no such row is found (including the special
case where the subquery returns no rows).
As usual, null values in the rows are combined per
the normal rules of SQL Boolean expressions. Two rows are considered
equal if all their corresponding members are non-null and equal; the rows
are unequal if any corresponding members are non-null and unequal;
otherwise the result of that row comparison is unknown (null).
If there is at least one null row result, then the result of ANY
cannot be false; it will be true or null.
expression
operator
ALL (subquery
)
The right-hand side is a parenthesized
subquery, which must return exactly one column. The left-hand expression
is evaluated and compared to each row of the subquery result using the
given operator
, which must yield a Boolean
result.
The result of ALL
is “true” if all rows yield true
(including the special case where the subquery returns no rows).
The result is “false” if any false result is found.
NOT IN
is equivalent to <> ALL
.
Note that if there are no failures but at least one right-hand row yields
null for the operator's result, the result of the ALL
construct
will be null, not true.
This is in accordance with SQL's normal rules for Boolean combinations
of null values.
As with EXISTS
, it's unwise to assume that the subquery will
be evaluated completely.
row_constructor
operator
ALL (subquery
)
The left-hand side of this form of ALL
is a row constructor,
as described in Section 4.2.11, “Row Constructors”.
The right-hand side is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result,
using the given operator
. Presently,
only =
and <>
operators are allowed
in row-wise ALL
queries.
The result of ALL
is “true” if all subquery rows are equal
or unequal, respectively (including the special
case where the subquery returns no rows).
The result is “false” if any row is found to be unequal or equal,
respectively.
As usual, null values in the rows are combined per
the normal rules of SQL Boolean expressions. Two rows are considered
equal if all their corresponding members are non-null and equal; the rows
are unequal if any corresponding members are non-null and unequal;
otherwise the result of that row comparison is unknown (null).
If there is at least one null row result, then the result of ALL
cannot be true; it will be false or null.
row_constructor
operator
(subquery
)
The left-hand side is a row constructor,
as described in Section 4.2.11, “Row Constructors”.
The right-hand side is a parenthesized subquery, which must return exactly
as many columns as there are expressions in the left-hand row. Furthermore,
the subquery cannot return more than one row. (If it returns zero rows,
the result is taken to be null.) The left-hand side is evaluated and
compared row-wise to the single subquery result row.
Presently, only =
and <>
operators are allowed
in row-wise comparisons.
The result is “true” if the two rows are equal or unequal, respectively.
As usual, null values in the rows are combined per the normal rules of SQL Boolean expressions. Two rows are considered equal if all their corresponding members are non-null and equal; the rows are unequal if any corresponding members are non-null and unequal; otherwise the result of the row comparison is unknown (null).