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“sqlserver”三表联如何查询“sql”语句？

假设学生表叫student,课程表叫class,选课表叫choose

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