FMS not WOrking

Hi

Below Fms not working. I want to implement when ItemCde changes or u_Type value changes

SELECT 
CASE
WHEN U_Type = 'U_1' then (Select U_1 from OITM where OITM.ItemCode =  $[RDR1.ITEMCODE])
WHEN U_Type = 'U_2' then (Select U_2 from OITM where OITM.ItemCode =  $[RDR1.ITEMCODE])
END AS Type
FROM [Test].[dbo].[RDR1]
where docentry=$[$Rdr1.docentry.number]

Thanks