Mysql 多表查询
内连接：
两张表共有的部分(内连接)，取交集
select * from student t1
inner join st t2 on t1.s_id=t2.s_id
左连接：
A独有的部分加上和A和B公共 的部分。也叫左外连接。
select * from student t1
left join st t2 on t1.s_id=t2.s_id
这张图就是A表独有的部分。
select * from student t1
left join st t2 on t1.s_id=t2.s_id
where t2.s_id is null
右连接：
b独有的部分加上和A和B公共 的部分。也叫右外连接。
select * from student t1
right join st t2 on t1.s_id=t2.s_id
这张图是B表独有的部分。
select * from student t1
right join st t2 on t1.s_id=t2.s_id
where t1.s_id is null
全连接：
select * from student t1
left join st t2 on t1.s_id=t2.s_id
union all
select * from student t1
right join st t2 on t1.s_id=t2.s_id
全外连接：
select * from student t1
left join st t2 on t1.s_id=t2.s_id
where t2.s_id is null
union all
select * from student t1
right join st t2 on t1.s_id=t2.s_id
where t1.s_id is null
