1.case用法

-- 用法一：

case when 条件1 then 字段取值1

when 条件2 then 字段取值2

when 条件3 then 字段取值3

else 字段取值4-- 如果上述全部不满足，则执行

end

-- 用法二：

case 字段名

when 取值1 then 字段取值1

when 取值2 then 字段取值2

when 取值3 then 字段取值3

else 字段取值4-- 如果上述全部不满足，则执行

end

2.if用法

if(条件,取值1,取值2)--若条件满足，则取值1，否则取值2

例子：现有用户行为数据表etl_online，查找2020年12月1日-2020年12月15日文章的下发数量，下发未曝光数量，曝光未点击数量，点击数量：

create table etl_online(

user_id varchar(20),-- 用户id

doc_id varchar(20),-- 文章id

auction varchar(20),-- 用户行为

p_day varchar(20)-- 时间

)

select p_day,doc_id,

case when click>0 then '点击'

when click<0 and rview>0 then '曝光未点击'

when rview<0 and vview>0 then '下发未点击'

when view<0 then '未下发'

else '其他' end as user_action,

count(user_id) as usrnum

(select p_day,doc_id,user_id, sum(click) as click, sum(view) as vview, sum(review) as rview

from

(select p_day,doc_id,user_id

if (uaction ='CLICK',1,0) as click,

if(uaction='VIEW',1,0) as vview,

if(uaction='RVIEW',1,0) as rview

from eta_online

where p_day between '2020-12-01' and '2020-12-15'

group by 1,2,3,4,5,6) as a

group by 1,2,3

) as a

group by 1,2,3

再来一个例子：

查询语文成绩优秀(85以上),良好(70-85)，及格(60-70)的人数：

方法一：

select GP,count(GP) as num from

(select stu_id,

case when grade>=85 then'优秀'

when grade>=70 and grade <85 then '良好'

when grade<70 and grade >=60 then '及格'

else '不及格' end as GP

from

grade

join

(select couid from course where couname='语文') as a

on a.couid=grade.couid)

group by 1

方法二：

select count(if(grade>=85,stu_id,null)) as A,

count(if(grade>=70 and grade<85,stu_id,null)) as B,

count(if(grade>=60 and grade<70,stu_id,null)) as C,

count(if(grade<60,stu_id,null)) as D

from grade

join

(select couid from course where couname='语文') as a

on grade.stu_id=a.stu_id