แก้ไขอย่างไรครับมันไม่รู้จัก Case when เวลาที่ใส่ where ครับ

select a.* from (
    SELECT Cust.*,
    CASE
    WHEN Staffvacation.T_date=curdate() and Staffvacation.Branchcode = Cust.Branchname THEN Staffvacation.Staff1
    ELSE Cust.Salecode
    END AS Staff
    FROM Cust LEFT JOIN Staffvacation
    on Staffvacation.Branchcode = Cust.Branchname
) as a
where a.Staff='$UT' and a.CustID = '{$_POST["vardata"]}'

select if( <conditiion>,  <do this if true>, <do this is false> ) as var_name