MyBatis---join 查询

Posted 2020-10-20 取什么昵称呢

在实际业务中，经常能碰到多表关联查询

下面的Demo，讲举例join查询在MyBatis中的实现

User 类：

package com.zy.domain;

import java.io.Serializable;
import java.util.List;

/**
 * @author gacl
 * users表所对应的实体类
 */
public class User  implements Serializable{

    //实体类的属性和表的字段名称一一对应
    private int id;
    private String name;
    private int age;
    private List<Role> role;

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    
    public List<Role> getRole() {
        return role;
    }

    public void setRole(List<Role> role) {
        this.role = role;
    }

    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + ", age=" + age + "]";
    }
}

Role 类：

package com.zy.domain;

import java.io.Serializable;

public class Role implements Serializable{

    private int rid;
    private int userId;
    private String roleName;
    public int getRid() {
        return rid;
    }
    public void setRid(int rid) {
        this.rid = rid;
    }
    public int getUserId() {
        return userId;
    }
    public void setUserId(int userId) {
        this.userId = userId;
    }
    public String getRoleName() {
        return roleName;
    }
    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }
    
    
}

Mapping配置文件：

<?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">
<!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
例如namespace="me.gacl.mapping.userMapper"就是me.gacl.mapping(包名)+userMapper(userMapper.xml文件去除后缀)
 -->
<mapper namespace="com.zy.mapping.userMapper">
    <!-- 在select标签中编写查询的SQL语句， 设置select标签的id属性为getUser，id属性值必须是唯一的，不能够重复
    使用parameterType属性指明查询时使用的参数类型，resultType属性指明查询返回的结果集类型
    resultType="me.gacl.domain.User"就表示将查询结果封装成一个User类的对象返回
    User类就是users表所对应的实体类
    -->
    <resultMap id="queryForListMap" type="com.zy.domain.User">  
        <id column="id" property="id" jdbcType="INTEGER"/>  
        <result column="name" property="name" jdbcType="VARCHAR"/>  
        <result column="age" property="age" jdbcType="INTEGER"/>  
        <collection property="role" javaType="java.util.List" ofType="com.zy.domain.Role">  
            <id column="rid" property="rid" jdbcType="INTEGER" />  
            <result column="userId" property="userId" jdbcType="VARCHAR" />  
            <result column="roleName" property="roleName" jdbcType="VARCHAR" />  
        </collection>  
    </resultMap>
    <select id="queryForList" resultMap="queryForListMap">  
        SELECT  
          u.id,  
          u.name,  
          u.age,  
          r.rid,  
          r.userId,  
          r.roleName 
        FROM  
          users u  
        LEFT JOIN  
          role r  
        ON  
          u.id = r.userid  
    </select> 
</mapper>

Test类：

package com.zy.test;

import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.util.List;

import com.zy.domain.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class Test1 {

    public static void main(String[] args) throws IOException {
        //mybatis的配置文件
        String resource = "config.xml";
        //使用类加载器加载mybatis的配置文件（它也加载关联的映射文件）
        InputStream is = Test1.class.getClassLoader().getResourceAsStream(resource);
        //构建sqlSession的工厂
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
        //使用MyBatis提供的Resources类加载mybatis的配置文件（它也加载关联的映射文件）
        //Reader reader = Resources.getResourceAsReader(resource); 
        //构建sqlSession的工厂
        //SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
        //创建能执行映射文件中sql的sqlSession
        SqlSession session = sessionFactory.openSession();
        /**
         * 映射sql的标识字符串，
         * me.gacl.mapping.userMapper是userMapper.xml文件中mapper标签的namespace属性的值，
         * getUser是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
         */
        /*String statement = "com.zy.mapping.userMapper.getUser";//映射sql的标识字符串
        //执行查询返回一个唯一user对象的sql
        User user = session.selectOne(statement, 1);*/
        /*String statement = "com.zy.mapping.userMapper.addUser";
        User user = new User();
        user.setName("张三");
        user.setAge(20);
        int user2 = session.insert(statement, user);
        session.commit();
        session.close();
        System.out.println(user2);*/
        //updateUser(session);
        //deleteUser(session);
        queryList(session);
    }

    
    public static void queryList(SqlSession session){
        String statement = "com.zy.mapping.userMapper.queryForList";
        List<Object> resultList = session.selectList(statement);
        System.out.println(resultList);
    }
}