多表查询：
显示内连接：
select 字段列表 from 表名1 inner join 表名1 on 条件
* inner 可忽略
select * from student inner join class on student.clas_id=class.id
隐式内连接：
使用where:
select t1.name,t2.name from student t1,class t2 where t1.class_id=t2.id
外连接：
左外链接
右外链接
*outer一般忽略
select * from student left join outer class on student.clas_id=class.id
子查询：
select *from student where class_id = (select max(class_id) from student)
select *from student where class_id in (select id from student where id=2 or id =3)
select *from student t1 ,(select *from class where class.id>2) where t1.class_id=t2.id