By specifying a NULL predicate, (null_predicate) you can test whether the value is a NULL value.
SQL Tutorial, Negative Conditions: NOT
The truth content of a NULL predicate is either true or false.
x |
Result of the Function x IS NULL |
---|---|
x is a NULL value |
True |
x is a special NULL value |
False |
x IS NOT NULL has the same result as NOT(x IS NULL).