如何在SQL Server Express 2008中以可執行方式編寫以下查詢

Question

select doc_no,date,acc_no,amount, 
if xvar=1 
(select acc_head from my_crem c where c.acc_no=o.acc_no) as acc_head 
else 
(select acc_head from my_debm c where c.acc_no=o.acc_no) as acc_head 
from my_ordm o; 

在上述查詢中，xvar不是列名。這是一個局部變量名稱。那我怎麼寫上面的查詢呢？

Answer 1

select 
    o.doc_no, 
    o.date, 
    o.acc_no, 
    o.amount, 
    CASE 
     WHEN @xvar = 1 THEN c.acc_head 
     ELSE d.acc_head 
    END as acc_head 
from 
    my_ordm o 
    left join my_crem c on c.acc_no = o.acc_no 
    left join my_debm d on d.acc_no = o.acc_no; 

Answer 2

select 
    doc_no, 
    date, 
    acc_no, 
    amount, 
    Case @xvar 
      WHEN 1 Then (select acc_head from my_crem c where c.acc_no = o.acc_no) 
      ELSE (select acc_head from my_debm c where c.acc_no = o.acc_no) 
    END AS acc_head 
from my_ordm o;