IF-Else condition in SQL query

In Expression, you can use the CASE Statements to acieve the IF ELSE IF functionality

CASE WHEN Source1.FIELD1 IS NULL THEN Source2.FIELD1
WHEN Source2.FIELD1 IS NULL THEN Source3.FIELD1
ELSE Source1.FIELD1
END

Hope you can do now.
Regards,
N. Boralkar
India