SQL Server / Oracle NULLIF Function

 NULLIF FUNCTION:

Compares two values and does the following:

Returns null if both values are the same.
Returns the first value if both values are different.

SQL Server:
Select NULLIF(FIELD1, FIELD2) AS FIELD3 FROM TABLE;

Oracle:
Select NULLIF(FIELD1, FIELD2) AS FIELD3 FROM TABLE;