This section describes the SQL-compliant subquery expressions available in Hyper. All of the expression forms documented in this section return Boolean results.

`EXISTS`

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 long enough to determine whether at least one row is returned, not all the way to completion. For this reason, it is unwise to write a subquery that has side effects, since those side effects may or may not occur depending on the results of the subquery.

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 unimportant. A common coding convention is
to write all `EXISTS`

expressions in the form
`EXISTS(SELECT 1 WHERE ...)`

.
However, there are exceptions to
this rule, such as subqueries that use `INTERSECT`

.
In that case, a `SELECT 1`

in all operands will cause the `EXISTS`

expression to
always evaluate to `true`

. The same holds for
`EXCEPT`

, with the `EXISTS`

evaluating to
`false`

in that case.

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 several matching `tab2`

rows:

SELECT col1 FROM tab1 WHERE EXISTS (SELECT 1 FROM tab2 WHERE col2 = tab1.col2);

`IN`

IN (`expression`

)`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
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.

`NOT IN`

NOT IN (`expression`

)`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 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.

`ANY`

/`SOME`

`expression`

ANY (`operator`

)`subquery`

`expression`

SOME (`operator`

)`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
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.

`ALL`

`expression`

ALL (`operator`

)`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 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`

.
`NOT IN`

is equivalent to `<> ALL`

.

As with `EXISTS`

, it's unwise to assume that the subquery will
be evaluated completely.