多表查询

1 内连接

select * from a inner join b on a.a_id = b.a_id;

也可以这样写

select * from a,b where a.a_id = b.b_id

2左外连接

select * from a left (out) join b on a.a_id = b.a_id  -----认为a是左

3右外连接

select * from a right (outer) join b on a.a_id = b.a_id;

4全外连接

select * from a full outer join b on a.a_id = b.b_id;