sqlserver三表联查sql语句

假设学生表叫student,课程表叫class,选课表叫choose

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1.三层嵌套的问题

select student.name from student where student.id IN

(select choose.sid from choose where choose.cid NOT IN

(select class.id from class where class.teacher='李明'))

2.一个内连接，一个嵌套

select student.name,avg(choose.score) from

student inner join choose on student.id=choose.sid

where student.id IN

(select choose.sid from choose

where choose.score'60'

group by choose.sid

having count(choose.sid)=2)

gruop by student.id

3.一个联合查询，一个嵌套查询

select student.name from student

where student.id IN

(select c1.sid from choose c1 where choose.cid='1'

union

select c2.sid from choose c2 where choose.cid='2'

on c1.sid=c2.sid

)

4.其实就是自连接查询和行列交换的问题：

select student.id,

(case choose.id when '1' then choose.score end) as 1号课成绩,

(case choose.id when '2' then choose.score end) as 2号课成绩,

from student inner join choose on student.id=choose.sid sc1,

student inner join choose on student.id=choose.sid sc2

where sc1.id='1'

and sc2.id='2'

and sc1.scoresc2.score

sqlserver中的关联查询问题

sql server 本省对语句就有自动优化功能， 第一个里边where语句相当于join on 来操作的

这样看来两个的效率基本上是一样的，你可以做两个表试一试。但是join的写法有助于你的编写语法检查，和易读性

sqlserver联合查询问题

使用 case when ..then else end 语句进行显示

多列数据展示都是使用case end

学会使用case end可以展示多列报表

自己试试吧

sqlserver多表联合查询

select a.a_name as 名字,count(b.a_id) as 数量 from a inner join b on a.a_id = b.a_id group by a.a_name

名字 数量

me 3

wo 1

he 1

select a.a_name as 名字,count(b.a_id) as 数量 from a left join b on a.a_id = b.a_id group by a.a_name

名字 数量

me 3

wo 1

he 1

she 0

our 0

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