case when 条件1 then 结果1
when 条件2 then 结果2
when 条件3 then 结果3
else 结果4 end
这就相当于if
else if
else if
else
select device_id, gender, (case when age >= 20 and age < 25 then "20-24岁" when age < 20 then "20岁以下" when age >= 25 then '25岁及以上' else '其他' end ) as age_cut from user_profile