mysql中 case when ··· then ··· when ··· then ··· else ··· 的使用方法

select *,(CASE WHEN ((SELECT now()) BETWEEN start_time and end_time) THEN '中间' when ((SELECT now()) < start_time) then '之前' ELSE '结束' END) as paper_state
from order_paper op where createUser = 1207 and Introduction like '%开始%';