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 (*value*[, ...])

The right-hand side of this form of IN is a parenthesized list
of scalar expressions. The result is TRUE if the left-hand expression's
result is equal to any of the right-hand expressions. This is a shorthand
notation for

*expression* = *value1*
OR
*expression* = *value2*
OR
...

Note that if the left-hand expression yields NULL, or if there are
no equal right-hand values and at least one right-hand expression 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.

*expression* IN (*subquery*)

The right-hand side of this form of IN 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.

(*expression*, *expression*[, ...]) IN (*subquery*)

The right-hand side of this form of IN is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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, NULLs in the expressions or subquery 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 (*value*[, ...])

The right-hand side of this form of NOT IN is a parenthesized list
of scalar expressions. The result is TRUE if the left-hand expression's
result is unequal to all of the right-hand expressions. This is a shorthand
notation for

*expression* <> *value1*
AND
*expression* <> *value2*
AND
...

Note that if the left-hand expression yields NULL, or if there are
no equal right-hand values and at least one right-hand expression yields
NULL, the result of the NOT IN construct will be NULL, not TRUE
as one might naively expect.
This is in accordance with SQL's normal rules for Boolean combinations
of NULL values.

*expression* NOT IN (*subquery*)

The right-hand side of this form of NOT IN 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.

(*expression*, *expression*[, ...]) NOT IN (*subquery*)

The right-hand side of this form of NOT IN is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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, NULLs in the expressions or subquery 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 of this form of ANY 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.

(*expression*, *expression*[, ...]) *operator* ANY (*subquery*)
(*expression*, *expression*[, ...]) *operator* SOME (*subquery*)

The right-hand side of this form of ANY is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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 queries.
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, NULLs in the expressions or subquery 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 of this form of ALL 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.

(*expression*, *expression*[, ...]) *operator* ALL (*subquery*)

The right-hand side of this form of ALL is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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, NULLs in the expressions or subquery 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.

(*expression*, *expression*[, ...]) *operator* (*subquery*)
(*expression*, *expression*[, ...]) *operator* (*expression*, *expression*[, ...])

The left-hand side is a list of scalar expressions. The right-hand side
can be either a list of scalar expressions of the same length, or a
parenthesized subquery, which must return exactly as many columns as there
are expressions on the left-hand side. 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, or to the right-hand expression list.
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, NULLs in the expressions or subquery 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).