case when as将外键引用的编号用sql语句转换成能看懂的说明

例子：
select Id,FromUserId,ToUserId,[Message],RecordTime,MessageState,
case
when MessageState=1 then '就'
when MessageState=2 then '在'
when MessageState=0 then '中'
end as 状态
from message

as是将修改列名