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SELECT s.*,p.player_name FROM t_match_score s
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LEFT JOIN t_match_player p
ON s.player_id=p.player_id
select c.discount
from a,b,c where a.hw_name=b.hw_name
and b.sort_id=c.sort_id
and a.hw_name='苹果'
假设学生表叫student,课程表叫class,选课表叫choose\x0d\x0a1.三层嵌套的问题\x0d\x0aselect student.name from student where student.id IN\x0d\x0a (select choose.sid from choose where choose.cid NOT IN\x0d\x0a (select class.id from class where class.teacher='李明'))\x0d\x0a2.一个内连接，一个嵌套\x0d\x0aselect student.name,avg(choose.score) from\x0d\x0astudent inner join choose on student.id=choose.sid\x0d\x0awhere student.id IN\x0d\x0a (select choose.sid from choose \x0d\x0a where choose.score=2)\x0d\x0agruop by student.id\x0d\x0a3.一个联合查询，一个嵌套查询\x0d\x0aselect student.name from student\x0d\x0awhere student.id IN\x0d\x0a(select c1.sid from choose c1 where choose.cid='1'\x0d\x0a union \x0d\x0a select c2.sid from choose c2 where choose.cid='2'\x0d\x0a on c1.sid=c2.sid\x0d\x0a)\x0d\x0a4.其实就是自连接查询和行列交换的问题：\x0d\x0aselect student.id,\x0d\x0a(case choose.id when '1' then choose.score end) as 1号课成绩,\x0d\x0a(case choose.id when '2' then choose.score end) as 2号课成绩,\x0d\x0afrom student inner join choose on student.id=choose.sid sc1,\x0d\x0astudent inner join choose on student.id=choose.sid sc2\x0d\x0awhere sc1.id='1' \x0d\x0aand sc2.id='2'\x0d\x0aand sc1.scoresc2.score