mysql连表查询,多表查询

先展示两个表

学生表：

老师表：

年级班级表

普通连表查询

查询年级的管理老师和班级的管理老师
select gradeclass.name as gradeName,teacher.name as teacherName from gradeclass,teacher where gradeclass.leaderid = teacher.id

查询多个表使用 select 字段名,字段名… from 表名1,表名2 的方式查询两张表

内连接查询 inner join

inner 可以省略
查询年级的管理老师和班级的管理老师
select gradeclass.name as gradeName,teacher.name as teacherName from gradeclass inner join teacher on gradeclass.leaderid = teacher.id

查询年级的年级主任老师
select gradeclass.name as gradeName,teacher.name as teacherName from gradeclass inner join teacher on gradeclass.leaderid = teacher.id where gradeclass.pid = 0

inner join也可以本表连接本表(这样需要起别名)
查询每个年级的班级，显示年级和班级名称
select g1.name as grade ,g2.name as class from gradeclass g1 join gradeclass g2 on g1.id=g2.pid

注：inner join 方式的使用比普通连表查询对比：

查询效率 inner join 要快很多 (因为使用普通查询where 的方式查询的时候所全不匹配一遍，然而inner join on 的方式on只匹配了匹配的内容)
普通查询的方式where后面就没有了，而inner join on 后面还可以跟where再次过滤数据

外连接 left join on 和 right join on

left join 就是把左面的表认为成主表
right join 就是把右面的表认为成主表

查询年级和班级的负责老师
select gradeclass.name as gradeName,teacher.name as teacherName from gradeclass right join teacher on gradeclass.leaderid = teacher.id

因为right join是右连接所以后面的表为主宋老师因为没有管理班级或年级所以年级班级位置为null

上面的查询代码直接换成left join就会以左面的表认为成主表显示
select gradeclass.name as gradeName,teacher.name as teacherName from gradeclass left join teacher on gradeclass.leaderid = teacher.id

上面的sql语句就是改了一下left join左连接，因为左连接是左面的表为主表宋老师的名字直接不显示

多表连查

查询年级下班级的班主任，显示成几年级几班，老师只显示班主任
select concat(g1.name,g2.name) gradeclassname,t.name from gradeclass g1 join gradeclass g2 on g1.id=g2.pid left join teacher t on g2.leaderid = t.id

查询年级下班级的班主任和学生，显示成几年级几班，老师只显示班主任
select concat(g1.name,g2.name) gradeclassname,t.name teachername,s.name studentname from gradeclass g1 join gradeclass g2 on g1.id=g2.pid left join teacher t on g2.leaderid = t.id left join student s on s.grade=g2.id