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### 7.16.2 IN

expressionIN (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_constructorIN (subquery)

The left-hand side of this form of `IN`

is a row constructor,
as described in section 2.2.11 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 per-row results are either unequal or null, with at least one
null, then the result of `IN`

is null.

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