NULL value in functions, expressions and predicates

The keyword NULL cannot be specified for values in expressions (except in CASE and CAST expressions), functions and predicates. You can, however, specify subexpressions (for example, a column name) whose result is the NULL value.

If the NULL value occurs in an expression, the result of the expression is also the NULL value.

If the NULL value occurs in a predicate, the result is usually the truth value unknown. There are, however, exceptions such as the predicate IS [NOT] NULL, for example. The result of each function, operator and predicate if an operand is the NULL value can be found in the chapter “Compound language constructs”.