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The argument of exists is an arbitrary 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 long enough to determine whether at least one row is returned, not all the way to completion.

The result is true if the left-hand expression's result is equal to any of the right-hand expressions.

>>> printSQL $ true `in_` [true, false, null_]
TRUE IN (TRUE, FALSE, NULL)

The result is true if the left-hand expression's result is not equal to any of the right-hand expressions.

>>> printSQL $ true `notIn` [false, null_]
TRUE NOT IN (FALSE, NULL)

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 subAll is true if all rows yield true (including the case where the subquery returns no rows). The result is false if any false result is found. The result is null_ if no comparison with a subquery row returns false, and at least one comparison returns null_.

>>> printSQL $ subAll true (.==) (values_ (true `as` #foo))
(TRUE = ALL (SELECT * FROM (VALUES (TRUE)) AS t ("foo")))

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 subAny is true if any true result is obtained. The result is false if no true result is found (including the case where the subquery returns no rows).

>>> printSQL $ subAny "foo" like (values_ ("foobar" `as` #foo))
(E'foo' LIKE ANY (SELECT * FROM (VALUES (E'foobar')) AS t ("foo")))