PostgreSQL 9.1.18 Documentation | ||||
---|---|---|---|---|

Prev | Up | Chapter 9. Functions and Operators | Next |

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.

expressionIN (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=value1ORexpression=value2OR ...

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.

expressionNOT 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<>value1ANDexpression<>value2AND ...

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 yis equivalent toNOT (x IN y)in all cases. However, null values are much more likely to trip up the novice when working withNOT INthan when working withIN. It is best to express your condition positively if possible.

expressionoperatorANY (array expression)expressionoperatorSOME (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 case where the array has zero elements).

If the array expression yields a null array, the result of
`ANY` will be null. If the left-hand
expression yields null, the result of `ANY` is ordinarily null (though a non-strict
comparison operator could possibly yield a different result).
Also, if the right-hand array contains any null elements and no
true comparison result is obtained, the result of `ANY` will be null, not false (again, assuming a
strict comparison operator). This is in accordance with SQL's
normal rules for Boolean combinations of null values.

`SOME` is a synonym for `ANY`.

expressionoperatorALL (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 case where the array has
zero elements). The result is "false" if any false result is found.

If the array expression yields a null array, the result of
`ALL` will be null. If the left-hand
expression yields null, the result of `ALL` is ordinarily null (though a non-strict
comparison operator could possibly yield a different result).
Also, if the right-hand array contains any null elements and no
false comparison result is obtained, the result of `ALL` will be null, not true (again, assuming a
strict comparison operator). This is in accordance with SQL's
normal rules for Boolean combinations of null values.

row_constructoroperatorrow_constructor

Each side is a row constructor, as described in Section
4.2.13. The two row values must have the same number of
fields. Each side is evaluated and they are compared row-wise.
Row comparisons are allowed when the `operator` is `=`,
`<>`, `<`, `<=`, `>` or `>=`, or has
semantics similar to one of these. (To be specific, an operator
can be a row comparison operator if it is a member of a B-tree
operator class, or is the negator of the `=` member of a B-tree operator class.)

The `=` and `<>` cases work slightly differently from
the others. 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).

For the `<`, `<=`, `>` and
`>=` cases, the row elements are
compared left-to-right, stopping as soon as an unequal or null
pair of elements is found. If either of this pair of elements
is null, the result of the row comparison is unknown (null);
otherwise comparison of this pair of elements determines the
result. For example, `ROW(1,2,NULL) <
ROW(1,3,0)` yields true, not null, because the third pair
of elements are not considered.

Note:Prior to PostgreSQL 8.2, the<,<=,>and>=cases were not handled per SQL specification. A comparison likeROW(a,b) < ROW(c,d)was implemented asa < c AND b < dwhereas the correct behavior is equivalent toa < c OR (a = c AND b < d).

row_constructorIS DISTINCT FROMrow_constructor

This construct is similar to a `<>` row comparison, but it does not yield
null for null inputs. Instead, any null value is considered
unequal to (distinct from) any non-null value, and any two
nulls are considered equal (not distinct). Thus the result will
either be true or false, never null.

row_constructorIS NOT DISTINCT FROMrow_constructor

This construct is similar to a `=`
row comparison, but it does not yield null for null inputs.
Instead, any null value is considered unequal to (distinct
from) any non-null value, and any two nulls are considered
equal (not distinct). Thus the result will always be either
true or false, never null.

Note:The SQL specification requires row-wise comparison to return NULL if the result depends on comparing two NULL values or a NULL and a non-NULL. PostgreSQL does this only when comparing the results of two row constructors or comparing a row constructor to the output of a subquery (as in Section 9.20). In other contexts where two composite-type values are compared, two NULL field values are considered equal, and a NULL is considered larger than a non-NULL. This is necessary in order to have consistent sorting and indexing behavior for composite types.