

PostgreSQL 8.2.9 Documentation  

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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 SQLcompliant. All of the expression forms documented in this section return Boolean (true/false) results.
expression IN (value [, ...])
The righthand side is a parenthesized list of scalar expressions. The result is "true" if the lefthand expression's result is equal to any of the righthand expressions. This is a shorthand notation for
expression = value1 OR expression = value2 OR ...
Note that if the lefthand expression yields null, or if there are no equal righthand values and at least one righthand 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.
expression NOT IN (value [, ...])
The righthand side is a parenthesized list of scalar expressions. The result is "true" if the lefthand expression's result is unequal to all of the righthand expressions. This is a shorthand notation for
expression <> value1 AND expression <> value2 AND ...
Note that if the lefthand expression yields null, or if there are no equal righthand values and at least one righthand 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.
expression operator ANY (array expression) expression operator SOME (array expression)
The righthand side is a parenthesized expression, which must yield an array value. The lefthand 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).
If the array expression yields a null array, the result of ANY will be null. If the lefthand expression yields null, the result of ANY is ordinarily null (though a nonstrict comparison operator could possibly yield a different result). Also, if the righthand 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.
expression operator ALL (array expression)
The righthand side is a parenthesized expression, which must yield an array value. The lefthand 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.
If the array expression yields a null array, the result of ALL will be null. If the lefthand expression yields null, the result of ALL is ordinarily null (though a nonstrict comparison operator could possibly yield a different result). Also, if the righthand 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_constructor operator row_constructor
Each side is a row constructor, as described in Section 4.2.11. The two row values must have the same number of fields. Each side is evaluated and they are compared rowwise. 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 BTree operator class, or is the negator of the = member of a BTree operator class.)
The = and <> cases work slightly differently from the others. Two rows are considered equal if all their corresponding members are nonnull and equal; the rows are unequal if any corresponding members are nonnull and unequal; otherwise the result of the row comparison is unknown (null).
For the <, <=, > and >= cases, the row elements are compared lefttoright, 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 like ROW(a,b) < ROW(c,d) was implemented as a < c AND b < d whereas the correct behavior is equivalent to a < c OR (a = c AND b < d).
row_constructor IS DISTINCT FROM row_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 nonnull value, and any two nulls are considered equal (not distinct). Thus the result will always be either true or false, never null.
row_constructor IS NOT DISTINCT FROM row_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 nonnull value, and any two nulls are considered equal (not distinct). Thus the result will always be either true or false, never null.