关于MyBatis一对多，多对一的xml语句的编写

1.首先是环境的配置

Pojo实体类的编写：

package Pojo;

import lombok.Data;
@Data
public class User {
    private int id;
    private String name;
    private String password;
    private Userboss userboss;


    public User(int id, String name, String password, Userboss userboss) {
        this.id = id;
        this.name = name;
        this.password = password;
        this.userboss = userboss;
    }

    public Userboss getUserboss() {
        return userboss;
    }

    public void setUserboss(Userboss userboss) {
        this.userboss = userboss;
    }

    public User() {

    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getPassword() {
        return password;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", password='" + password + '\'' +
                ", userboss=" + userboss +
                '}';
    }
}

package Pojo;

import lombok.Data;

import java.util.List;

@Data
public class Userboss {
    private int id;
    private String name;
    private String password;
    private List<User> users;
}

Dao包的编写，主要是接口Mapper和相应的xml文件的配置

 

 配套的多对一xml文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="Dao.UserbossMapper">
    <resultMap id="usermap2" type="Pojo.Userboss">
        <id column="tId" property="id" />
        <result column="tname" property="name"/>
        <result column="tpsw" property="password"/>
        <collection property="users"  ofType="Pojo.User">
          <result property="password" column="spsw"/>
            <result column="sId" property="id"/>
            <result property="name" column="sName"/>
        </collection>
    </resultMap>
    <select id="findall1" resultMap="usermap2">
        select t.id tId,t.name tname,t.psw tpsw,s.id sId,s.name sName ,s.psw spsw from usersboss t ,users s where t.id = s.tid
    </select>

</mapper>

 配置的一对多xml文件：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="Dao.UserMapper">

<select id="findall" resultMap="usermap1">
    SELECT s.id sid, s.name sname ,s.psw spsw, t.name tname,t.id tid,t.psw tpsw
    FROM users s,usersboss t
    WHERE s.tid = t.id

</select>
    <resultMap id="usermap1" type="Pojo.User">
        <id column="sid" property="id" />
        <result column="sname" property="name"/>
        <result column="spsw" property="password"/>
        <association property="userboss" javaType="Pojo.Userboss">
            <result property="name" column="tname"/>
            <result property="password" column="tpsw"/>
            <result property="id" column="tid"/>
        </association>
    </resultMap>

</mapper>

配套的数据库如下：

 

 

 

注意要进行两个表的联结

2.查询的说明

我这里都采用的是按照结果嵌套处理，同时还有这另外按照查询嵌套处理，这里不多介绍。

小结

1. 关联 - association【多对一】

2. 集合 - collection【一对多】

3. javaType & ofType

   • javaType 用来指定实体类中属性的类型

   • ofType 用来指定映射到List或者集合中pojo类型，泛型中的约束类型。

3.运行结果展示

测试程序：

 测试结果：

多对一：

 一对多：

 4.总结

注意点

• 保证sql的可读性，尽量保证通俗易懂

• 注意一对多和多对一中，属性和字段的问题

• 如果问题不好排查错误，可以使用日志，建议使用Log4j