使用CASE WHEN (2)

如何在where條件中使用Case when 呢?

看下面的例子: 

selec * from USER_TEST t1, ROLE_TEST t2, USER_ROLE t3
where 
(CASE WHEN t1.USER_ID <> '0' THEN t2.ROLE_ID ELSE t1.USER_ID END) = t3.USER_ID