mysql级联查询

Posted 2023-03-14

现有学生表 stu 字段sid sname
选课表 sid cname
现在需要查询选课最多的7人的姓名及选课的数目

参考技术A 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 参考技术B select a.sname,b.num from stu a,(

select count(*) num,sid from 选课表 group by sid
) b where a.sid = b.sid; 参考技术C 选课表为ss,
select stu.sname,count(ss.cname) as num from stu inner join ss on stu.sid=ss.sid group by ss.sid order by num asc

Mybatis + mysql 实现两级级联的查询

Mybatis + mysql 实现两级级联的查询

 

一、需求：根据Mybatis实现2级级联的查询。

根据省的id查询它下面所有的市信息。

二、实现步骤

 

方法一：

思想： 此种情况是“一对多”，我们在一的mapper.xml中，用collection定义多的一个集合即可。

property   ---> 对应实体类中，多的那个子类，list
ofType     ---> 对应子类的试题类型
column    ----> 父类传递给子类查询的外键
select      ----> 子类查询的方法

1.定义父类+子类的实体类

package com.imocc.mall.pojo;

import lombok.Data;

import java.util.List;

/**
 * @author wy
 */
@Data
public class Province {
    private Integer id;

    private String provinceid;

    private String province;

    private List<City> list;

    @Override
    public String toString() {
        return "Province{" +
                "id=" + id +
                ", provinceid=\'" + provinceid + \'\\\'\' +
                ", province=\'" + province + \'\\\'\' +
                ", list=" + list +
                \'}\';
    }
}

package com.imocc.mall.pojo;

import lombok.Data;

@Data
public class City {
    private Integer id;

    private String cityid;

    private String city;

    private String father;


}

2.mapper类

 

 2. mapperxml文件

 

 3. 测试

 

 

JDBC Connection [HikariProxyConnection@1532915766 wrapping com.mysql.cj.jdbc.ConnectionImpl@62dbe64e] will not be managed by Spring
==>  Preparing: select * from hat_province where provinceID =? 
==> Parameters: 110000(String)
<==    Columns: id, provinceID, province
<==        Row: 1, 110000, 北京市
====>  Preparing: select id, cityID, city, father from hat_city where father = ? 
====> Parameters: 110000(String)
<====    Columns: id, cityID, city, father
<====        Row: 1, 110100, 市辖区, 110000
<====        Row: 2, 110200, 县, 110000
<====      Total: 2
<==      Total: 1
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@8f2098e]
Province{id=1, provinceid=\'110000\', province=\'北京市\', list=[City(id=1, cityid=110100, city=市辖区, father=110000), City(id=2, cityid=110200, city=县, father=110000)]}