NULLIF is a miscellaneous function that returns a NULL value if
the arguments are equal.
Syntax

>>-NULLIF--(--expression1--,--expression2--)-------------------><
The NULLIF function returns a NULL value if the arguments are
equal; otherwise, it returns the value of the first argument. The arguments
must be comparable. The result of using
NULLIF(e1,e2) is
the same as using the expression:
CASE WHEN e1=e2 THEN NULL ELSE e1 END
When e1=e2 evaluates
to unknown (because one or both of the arguments is NULL), NULLIF returns
the value of the first argument.