Case When 的字段如何作为条件?

SELECT OrderNum=(case when item.docnum is not null then item.DocNum else ORDR.DocNum end).....
FROM  TB
INNER JOIN...
ON ...
WHERE 

我想把这个OrderNum 作为条件查询，需要怎么弄呢.,?
似乎

( case when item.docnum is not null then item.DocNum=1 else ORDR.DocNum=1 end)

这样不行
[解决办法]

( case when item.docnum is not null then item.DocNum else ORDR.DocNum end)=1

[解决办法]

SELECT .....
FROM TB
INNER JOIN...
ON ...
WHERE (case when item.docnum is not null then item.DocNum else ORDR.DocNum end)=XX