多表查询

、内连接、左连接、右连接

学生表：                      

班级表： 

 

1、内连接：inner join

select * from stu1 inner join class on stu1.grade=class.grade;

select s.*,c.id as c_id,c.grade as c_grade,room from stu1 as s inner join class as c on s.grade=c.grade;

2、左右连接

左链接：左表为主表，取出主表中所有记录，然后去匹配另一个表进行连接，匹配成功，保留数据；匹配不成功的话，另一个表的字段置为NULL.

右连接：道理同左链接

左表 + left ight + join + 右表 + on + 左表.字段 = 右表.字段;

select s.*,c.room from stu1 as s left join class as c on s.grade=c.grade;

delect s.*,c.room from stu1 as s right join class as c on s.grade=c.grade;