SQL Language Guide : 4. Elements of SQL Statements : Predicates in SQL : IS NULL Predicate
 
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IS NULL Predicate
Use IS NULL to determine whether an expression is null, because you cannot test for null by using the = comparison operator.
The IS NULL predicate takes the following form:
IS [NOT] NULL
For example:
x IS NULL
is true if x is a null.
IS UNKNOWN is a synonym for IS NULL when the expression is of the BOOLEAN type.