1.交叉连接（笛卡尔积）
select * from A, B
2.内连接
（1）隐式内连接
select * from A, B where b.aid = a.id
（2）显示内连接
select * from A inner join B on b.aid = a.id
3.外链接
（1）左外连接
select * from A left [outer] join B on b.aid = a.id
（2）右外连接
select * from A right [outer] join B on b.aid = a.id
HQL多表查询
1.内连接（迫切）
from Customer c inner join c.linkMans
from Customer c inner join fetch c.linkMans
2.外链接
（1）左外连接（迫切）
from Customer c left join fetch c.linkMans
（2）右外连接（迫切）
from Customer c right join fetch c.linkMans