mysql表的级联问题

mysql:

1、先给B表添加主键列

alter table tableB add constraint b_PK primary key(date,channel)

2、将B表的组合主键关联到A表的外键组合键中

alter table tableB add constraint tableB_fk foreign key(date,channel) references tableA (date,channel) on DELETE CASCADE

3 delete from tableA where date =? and channel =? 就出发相关记录··~

首先能实现目的的语法是这么写：

select id,name,a_rankfrom table_a,(select a_id,count(*) as a_rankfrom table_bgroup by a_id)where table_a.id = table_b.a_idorder by a_rank

学生表：stu。选课表：sc。代码如下已测试通过。

select stu.sname,count(sc.cname) as '选修总数目' from stu,sc where stu.sid=sc.sid group by stu.sname order by count(sc.cname) desc limit 0,7

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