The NULL Predicate
The NULL predicate is used to test if the specified expression is the NULL value, and has the form:
If the predicate specifies expression IS NULL, then the result is true if any operand in the expression is NULL.
The result is false if no operand in the expression is NULL.
The result of the NULL predicate is never unknown.
The use of composite expressions in NULL predicates provides a shorthand for testing whether any of the operands is NULL. Thus the predicate "A+B IS NULL" is an alternative to "A IS NULL OR B IS NULL", provided that A+B does not result in overflow.
Note: The actual arithmetical operator(s) used in numerical expressions in NULL predicates is irrelevant since all arithmetical operations involving a NULL value evaluate to the NULL value.
The NULL predicate is the only way to test for the presence of the NULL value in a column, since all other predicates where at least one of the operands is NULL evaluate to unknown.
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