请问：带case when的check约束如何写

请教：带case when的check约束怎么写
a表
returnflag=1时，验证amount=isnull(quantity,0)*isnull(realprice,0)
returnflag=0时，验证costamount=isnull(quantity,0)*isnull(costprice,0)

ALTER TABLE a
ADD CHECK 
(returnflag IN('0','1') and case returnflag when 0 then costamount=isnull(quantity,0)*isnull(costprice,0) when 1 then amount=isnull(quantity,0)*isnull(realprice,0) end )
这样写提示我：第 4 行: '=' 附近有语法错误。

[解决办法]

SQL code

ALTER TABLE aADD CHECK (    (returnflag=0 and costamount=isnull(quantity,0)*isnull(costprice,0))  or     (returnflag=1 and amount=isnull(quantity,0)*isnull(realprice,0)))
[解决办法]
探讨


引用:

SQL code
ALTER TABLE a
ADD CHECK
(costamount=
case returnflag
when 0 then isnull(quantity,0)*isnull(costprice,0)
when 1 then isnull(quantity,0)*isnull(realprice,0) end )

额，不是……