PostgreSQL 7.4beta4 Documentation
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# 9.17. Row and Array Comparisons

This section describes several specialized constructs for making multiple comparisons between groups of values. These forms are syntactically related to the subquery forms of the previous section, but do not involve subqueries. The forms involving array subexpressions are PostgreSQL extensions; the rest are SQL-compliant. All of the expression forms documented in this section return Boolean (true/false) results.

## 9.17.1. IN

    expression IN (value[, ...])

The right-hand side 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.

## 9.17.2. NOT IN

    expression NOT IN (value[, ...])

The right-hand side 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.

> Tip: x NOT IN y is equivalent to NOT (x IN y) in all cases. However, null values are much more likely to trip up the novice when working with `NOT IN` than when working with `IN`. It's best to express your condition positively if possible.

## 9.17.3. ANY/SOME (array)

    expression operator ANY (array expression)
    expression operator SOME (array expression)

The right-hand side is a parenthesized expression, which must yield an array value. The left-hand expression is evaluated and compared to each element of the array 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 array has zero elements).

`SOME` is a synonym for `ANY`.

## 9.17.4. ALL (array)

    expression operator ALL (array expression)

The right-hand side is a parenthesized expression, which must yield an array value. The left-hand expression is evaluated and compared to each element of the array using the given operator, which must yield a Boolean result. The result of `ALL` is "true" if all comparisons yield true (including the special case where the array has zero elements). The result is "false" if any false result is found.

## 9.17.5. Row-wise Comparison

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

Each side is a list of scalar expressions; the two lists must be of the same length. Each side is evaluated and they are compared row-wise. 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, 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 the row comparison is unknown (null).

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