（十九）连接查询

--1.简化连接查询
select s.id,s.name,c.id,c.name from studentBB s,classes c where s.cid=c.id;

--2、 left join左连接
--左连接
select s.id, s.name, c.id, c.name from studentB s left join classes c on s.cid = c.id;
 

--3、 right join右连接
--右连接
select s.id, s.name, c.id, c.name from studentB s right join classes c on s.cid = c.id;
 

--4、 inner join内连接
--内连接
select s.id, s.name, c.id, c.name from studentB s inner join classes c on s.cid = c.id;
 
--inner可以省略
select s.id, s.name, c.id, c.name from studentB s join classes c on s.cid = c.id;
 

--5、 cross join交叉连接
--交叉联接查询，结果是一个笛卡儿乘积
select s.id, s.name, c.id, c.name from studentB s cross join classes c
--where s.cid = c.id;
 

--6、 自连接（同一张表进行连接查询）
--自连接
select distinct s.* from studentB s, studentB s1 where s.id <> s1.id and s.sex = s1.sex;