MySQL多表连接查询

多表连接查询：

create table class(

   cid int primary key auto_increment,

   cname varchar(20)

)default charset=‘utf8‘;

create tablr stu(

   sid int primary key auto_increment,

   sname varchar(20).

   cid int

);default charset=‘utf8‘;

    insert into class(name) values(‘一班‘);

    insert into class(name) values(‘二班‘);

    insert into class(name) values(‘三班‘);

    insert into class(name) values(‘四班‘);

    insert into class(name) values(‘张三‘,1);

    insert into class(name) values(‘李四‘,2);

    insert into class(name) values(‘王五‘,2);

    insert into class(name) values(‘杨六‘,3);

    insert into class(name) values(‘赵七‘,5);

    insert into class(name) values(‘周八‘,6);

    select * from class;

    select * from stu;

-------------------------------------

    交叉连接：--没有任何条件，只需要将多张表的所有数据排列显示出来

select * from class,stu;

    内连接：建立在两张或多张表之间，存在数据的关联关系

inner join

    select * from 表A INNER JOIN 表B on 连接条件；

    等价于

    select * from 表A，表B where 表A.字段=表B.字段；

两张表：

select * from class INNER JOIN stu ON class.cid=stu.cid;

select * from class,stu, 表C WHERE class.cid=stu.cid AND 表A.字段=表C.字段；

三张表：

select * from student s,course c,score sc WHERE s.sid=sc.sid AND c.cid=sc.cid;

外连接：

左外连接 LEFT JOIN / LEFT OUTER JOIN

右外连接 RIGHT JOIN / RIGHT OUTER JOIN

外连接：左表或右表作为主表的情况下，主表的全部内容都会显示，而另一张表没有对应的数据则会补null

select * from class c LEFT JOIN stu s ON c.cid=s.cid;

select * from class c RIGHT JOIN stu s ON c.cid=s.cid;

MySQL多表连接查询