a = NULLas the result would be UNKNOWN. Instead use
a IS NULLor
a IS NOT NULLconditions. NULL is not equal to NULL. To compare two expressions where null can happen, use one of the functions described below. All operators except concatenation return NULL if one of their operand is NULL. For instance the result of
3 * NULL + 5is null.
NULL can't appear in columns restricted by a PRIMARY KEY or a NOT NULL constraint. (Exception is a new constraint with NOVALIDATE clause)