case 嵌套查询与连接查询你需要懂得

Case 子查询连接查询在sql开发中很常用的一个查询功能,感兴趣的你可不要错过本文，接下来为你详细介绍下

代码如下:
select * from score
create database demo
use demo
create table [user]
(
[uId] int identity( 1 ,1 ) primary key,
[name] varchar ( 50),
[level] int --1骨灰大虾菜鸟
)
insert into [user] ( name, level ) values (' 犀利哥 ', 1 )
insert into [user] ( name, level ) values (' 小月月 ', 2 )
insert into [user] ( name, level ) values (' 芙蓉姐姐 ', 3 )
--case end 单值判断 相当于 switch case
--then 后面的返回值类型必须一致
select [name] ,
case [level]
when 1 then '骨灰 '
when 2 then '大虾 '
when 3 then '菜鸟 '
end as '等级 '
from [user]
use MySchool
select * from score
--case end 第二种用法，相当于多重 if 语句
select studentId ,
case
when english >=90 then ' 优 '
when english >=80 and english when english >=70 and english when english >= 60 and english else ' 差 '
end as '成绩 '
from score
order by english

-- 表中有A B C 三列 ,用 SQL 语句实现：当 A列大于 B 列时选择A 列否则选择 B 列，当B 列大于 C列时选择 B 列否则选择 C列。
select
case
when a > b then a
else b
end ,
case
when b > c then b
else c
end
from T

-- 练习
create table test
(
number varchar ( 10),
amount int
)
insert into test( number ,amount ) values ( 'RK1', 10 )
insert into test( number ,amount ) values ( 'RK2', 20 )
insert into test( number ,amount ) values ( 'RK3',- 30 )
insert into test( number ,amount ) values ( 'RK4',- 10 )
select number ,
case
when amount > 0 then amount
else 0
end as '收入 ' ,
case
when amount else 0
end as '支出 '
from test
--结果如下