Subquery expressions v16
SQL-compliant subquery expressions are available in EDB Postgres Advanced Server. All of these expression forms return Boolean (TRUE/FALSE)
results.
EXISTS
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 that act as constants during any one evaluation of the subquery.
The subquery is generally executed only far enough to determine whether at least one row is returned, not all the way to completion. We recommend that you don't write a subquery that has any side effects (such as calling sequence functions). Whether the side effects occur might 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 deptno
, but it produces at most one output row for each dept
row, even though there are multiple matching emp
rows:
IN
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).
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 is NULL
, not FALSE
. This is in accordance with SQL’s normal rules for Boolean combinations of null values.
As with EXISTS
, you can't assume that the subquery will evaluate completely.
NOT IN
The right-hand side is a parenthesized subquery that 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.
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 is NULL
, not TRUE
. This is in accordance with SQL’s normal rules for Boolean combinations of null values.
As with EXISTS
, you can't assume that the subquery will evaluate completely.
ANY/SOME
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
.
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 is NULL
, not FALSE
. This is in accordance with SQL’s normal rules for Boolean combinations of null values.
As with EXISTS
, you can't assume that the subquery will evaluate completely.
ALL
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. The result is NULL
if the comparison doesn't return FALSE
for any row and it returns NULL
for at least one row.
NOT IN
is equivalent to <> ALL
. As with EXISTS
, you can't assume that the subquery will evaluate completely.