MySQL连表操作

MySQL连表

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连表操作

使userinfo表的part_id列与department的id关联

select * from userinfo,department where userinfo.part_id = department.id

推荐的连表方式 left/right join(连接，左连接/右连接，join是连接，left是从左边连，right是从右边连)

select * from userinfo left join department on userinfo.part_id = department.id

left是左边的userinfo表的全部显示，意味着department表中如果没有和userinfo相关的就不会被显示出来

select * from userinfo right join department on userinfo.part_id = department.id

right是右边的department表的全部显示，意味着userinfo表中如果没有和department相关的就不会被显示出来

innder方法，如果连表有一部分是null的就会给隐藏。

select * from userinfo innder join department on userinfo.part_id = department.id
userinfo表中没有和department表中相对应的值，会出现null，而innder方法将出现null时一行隐藏

统计表中某列的个数，用count统计。

select count(id) from userinfo;

关联多个表

select * from

　　userinfo

　　left join department on userinfo.part_id = department.id

　　left join department1 on userinfo.part_id = department.id

　　left join department2 on userinfo.part_id = department.id

　　left join department3 on userinfo.part_id = department.id

 前提是表和表之间是有关系

　　

例如：

　　select  score.sid, student.sid  from  score

　　　　left join student on score.student_id = student.sid

　　　　left join course on score.course_id = course.cid

　　　　left join class on student.class_id = class.cid

　　　　left join teacher on course.teacher_id=teacher.tid

select score_id,stu_id from score

　　left join student on score.stu_id = student.student_id

　　left join course on score.cou_id = course.course_id

　　left join class on student.cla_id = class.class_id

　　left join teacher on course.teach_id=teacher.teacher_id;

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