-- 查询出学生表和班级信息
select * from student,class where student.classid=class.classid
-- 5张表全部联查
select * from student,class,course,sc,teacher where student.classid=class.classid and student.sid=sc.sid
and sc.cid=course.cid and course.tid=teacher.tid
-- 查询学过张三老师课程的学生信息
select student.* from student,class,course,sc,teacher where student.sid=sc.sid
and sc.cid=course.cid and course.tid=teacher.tid
and teacher.tname='张三'
-- 多表最终跟单表一致
-- 查询每个学生的 学生信息，班级名称，平均成绩
select student.Sname,class.classid,avg(score) from student,class,sc
where student.sid=sc.sid and student.classid=class.classid
group by student.sid
-- 通过第一张表的结果进行on条件匹配
-- 表少 每张表的数据大，内存占用小， IO高
select * from student,class where student.classid=class.classid ssex='男'
1.3 inner join on
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表的个数多，每个表的数据量不大,吃内存 IO小，笛卡尔积
-- 通过第一张表的结果进行on条件匹配
select * from student inner join class on student.classid=class.classid
where ssex='男'
select * from student inner join class on student.classid=class.classid
inner join sc on student.sid=sc.sid -- 五表联查
select * from class inner join student on class.classid=student.classid
inner join sc on sc.sid=
-- 每门课程的平均成绩 课程名称 代课老师姓名 平均成绩
select cname,tname,avg(score) from sc
inner join course on sc.cid=course.cid
inner join teacher on course.tid=teacher.tid
group by course.cid
二、外联查询
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找到主查表
2.1 left join on 左外联
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主表在join左边
-- 所有学生的数据和对应的班级信息
-- 主表是student
select * from student
left join class on student.classid=class.classid
-- 查询出所有的学生学过多少门课程 学生姓名 课程数
select sname,count(cid) from student left join sc on student.sid=sc.sid
group by student.sid
-- 没有班级 的学生
select * from student left join class on student.classid=class.classid
where class.classid is null
-- 没有学生的班级
select * from class left join student on student.classid=class.classid
where student.sid is null
2.2 right join on 右外联
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主表在join右边
-- 没有学生的班级
select * from student
right join class on student.classid = class.classid
where student.sid is null
2.3 union
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两个结果集的并集
（1） 去除重复 distinct
（2）不同类型的字段是可以合并
（3）起别名给第一个结果集才有用
（4）不同列数量的结果集不能合并
-- 库中的所有人的名字
select sname,ssex,classid from student
union
select tname,tsex,temail from teacher
-- 获取没有学生的班级和没有班级的学生的数据
select * from student left join class on student.classid=class.classid
where class.classid is null
union
select * from student right join class on student.classid=class.classid
where student.sid is null
-- 获取没有班级的学生和、班级和学生都有、
-- 全连接
select * from student left join class on student.classid=class.classid
union
select * from student right join class on student.classid=class.classid
-- 不去重的并集
select * from student left join class on student.classid=class.classid
union all
select * from student right join class on student.classid=class.classid
三、子查询
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子查询，又称内部查询
3.1 where 子句
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--查询id最大的一个学生
select * from student order by sid desc limit 1
-- 查询id最大的一个学生（子查询）
-- 效率差
select max(sid) from student
select * from student where sid=(select max(sid) from student) -- 魔数
-- 查询每个班id最大的学生（子查询）
select * from student
left join class on student.classid=class.classid
where sid in(select max(sid) from student group by classid)
-- 查询学过张三老师课程的学生
select * from student where sid in(
select sid from sc where cid=
(select cid from course where tid=(
select tid from teacher where tname='张三')
)
)
-- 查询没学过张三老师课程的学生
select * from student where sid not in(
select sid from sc where cid=
(select cid from course where tid=(
select tid from teacher where tname='张三')
)
)
3.2 from 子查询
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查询结果将作为一张表使用
-- 查询大于5人的班级名称和人数（不使用子查询）
select classname,count(*) from class left join student on class.classid=student.classid
group by class.classid having count(*)>5
-- 查询大于5人的班级名称和人数（使用子查询）
select classname,人数 from class left join (
select classid,count(*) 人数
from student group by classid)t1
on class.classid = t1.classid
where 人数>5
3.3 exists 子查询
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子句有结果，父句执行，子句没结果，父句不执行
select * from teacher where exists(select * from student where classid=10)
3.4 any /some,all
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子查询
-- 查询一班成绩比二班最低分高的学生
SELECT DISTINCT student.* FROM sc
LEFT JOIN student ON sc.sid=student.sid
WHERE student.classid=1 AND score > ANY(
SELECT score FROM sc
LEFT JOIN student ON sc.sid=student.`Sid`
WHERE student.classid=2
)
-- all
SELECT DISTINCT student.* FROM sc
LEFT JOIN student ON student.sid=sc.sid
WHERE student.classid=1 AND score >ALL(
SELECT score FROM sc
LEFT JOIN student ON sc.sid=student.`Sid`
WHERE student.`classid`=2
)
-- expr1 字段
-- expr2 当字段为null时，默认值
select sid,sname,ifnull(birthday,'这个学生没有生日'),ssex from student
3.7 case when then end
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同时出现
select tid,tname,
case tsex
when 0 then '男'
when 1 then '女'
else '保密'
end tsex,tbirthday from teacher
select tid,tname,
case
when tsex>1 then '男'
when tsex=1 then '女'
when tsex<1 then '未知'
end,tbirthday from teacher
select score,sid,
case
when score>=90 then 'A'
when score>=80 then 'B'
when score>=70 then 'C'
when score>=60 then 'D'
when score<60 then '不及格'
end from sc