当B表有数据时---------- select count(*) from b 

select a1,a2 from  a  where  a1='**'  and  b1='**'

当B表没有数据时

select a1,a2 from  a  where  a1='**' 

这种不同情况查询条件不同的SQL怎么写，sqlserver。

我直接在where后面写case when 总是报错。

select * from RD_Craft where status != 0 and userright like '%,${fine_username},%' case when (select COUNT(*) from fine_user t1  left join  fine_user_role_middle t2 on t1.id = t2.userid  where t2.roleid  = 'a89c174d-5938-4a94-b642-f836af54cb33' and username ='${fine_username}') > 0 then "and 1=2" else "and 1=1" end

UNION all

select * from RD_Craft where status != 0  case when (select  COUNT(*) from fine_user t1  left join  fine_user_role_middle t2 on t1.id = t2.userid  where t2.roleid  = 'a89c174d-5938-4a94-b642-f836af54cb33' and username ='${fine_username}') > 0 then "and 1=1" else "and 1=2" end