MySQL连表操作
MySQL连表
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;
----- END -----
