NULLIF function

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

Syntax

Read syntax diagramSkip visual syntax diagramNULLIF (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.