NULLIF
Returns a null value if the two specified expressions are equal.
Syntax
NULLIF ( expression , expression )
Arguments
expression
Is any valid scalar expression.
Return types
Returns the same type as the first expression.
NULLIF returns the first expression if the two expressions are not equal. If the expressions are equal, NULLIF returns a null value of the type of the first expression.
Example
SELECT dept , NULLIF(current_year, previous_year) AS LastBudget FROM budgets;