case when then用法

--简单Case函数
CASE sex
         WHEN '' THEN '男'
         WHEN '' THEN '女'
ELSE '其他' END 

//案例
select cid as 渠道编号,
case cid   when '' then '华为'  when '' then '小米' else cid end as 渠道名称,   //别名  渠道名称，遇见3表示华为渠道，遇到20645表示小米渠道
       intdate as 注册日期,
       install as 激活人数,
       reg2 as 注册人数,
       concat(
  left(reg2 / install *, ), '%') as 注册率,
       ip as 去重IP数,
       device as 去重设备数,
       charge_user as 充值人数,
       charge_times as 充值次数,
       charge_money as 充值面额,
       charge_money2 as 新增充值面额,
       concat(
  left(day1 / reg2 *, ), '%') as 次日留存率,
       concat(
  left(day2 / reg2 *, ), '%') as 2日留存率,
       concat(
  left(day3/ reg2 *, ), '%') as 3日留存率,
       concat(
  left(day4 / reg2 *, ), '%') as 4日留存率,
       concat(
  left(day5 / reg2 *, ), '%') as 5日留存率,
       concat(
  left(day6 / reg2 *, ), '%') as 6日留存率,
       concat(
  left(day7 / reg2 *, ), '%') as 7日留存率
  from union_data
 where intdate>= ''
   and intdate<= ''