Mybatis關聯查詢之一對多和多對一XML配置詳解

平時在開發過程中dao、bean和XML文件都是自動生成的，很少寫XML的配置關系，今天記錄一下mybatis的關聯查詢中的多對一和一對多的情況。

首先是有兩張表(學生表Student和老師Teacher表)，為了更易懂，這里只設置了最簡單的幾個必要字段。表結構如下圖:

Student表：

​

Teacher表：

​

創建實體bean:

Teacher.java:

import java.util.List;
 
/**
 * TODO
 * @version 創建時間：2017年12月21日 上午9:02:45
 */
public class Teacher {
 
    private Integer id;
    private String name;
    private String className;
    private List<Student> students;
 
    public List<Student> getStudents() {
        return students;
    }
 
    public void setStudents(List<Student> students) {
        this.students = students;
    }
 
    public Integer getId() {
        return id;
    }
 
    public void setId(Integer id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public String getClassName() {
        return className;
    }
 
    public void setClassName(String className) {
        this.className = className;
    }
 
}

Student.java

/**
 * TODO
 * 
 * @author 作者 E-mail:[e-mail]
 * @version 創建時間：2017年12月21日 上午9:01:17
 */
public class Student {
 
    private Integer id;
    private String name;
    private Integer teacherId;
    private String className;
    private Teacher teacher;
    
 
    public Teacher getTeacher() {
        return teacher;
    }
 
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
 
    public Integer getId() {
        return id;
    }
 
    public void setId(Integer id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public Integer getTeacherId() {
        return teacherId;
    }
 
    public void setTeacherId(Integer teacherId) {
        this.teacherId = teacherId;
    }
 
    public String getClassName() {
        return className;
    }
 
    public void setClassName(String className) {
        this.className = className;
    }
 
    @Override
    public String toString() {
        return "{id:"+this.id+",name:"+this.name+",className:"+this.className+",teacherId:"+this.teacherId+"}";
    }
} 

　　下面重點來了：配置Mapper.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="com.tz.mybatis.dao.studentDao">  
    
    <!-- /////////////////////////////////一對多的第一種寫法，一般考慮到性能問題，不會這么實現//////////////////////// -->
    <resultMap type="Teacher" id="teacherMap">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <collection property="students" ofType="Student" column="id">
            <id column="sid" property="id"/><!-- 這里的column對應的是下面查詢的別名，而不是表字段名 -->
            <result column="sname" property="name"/><!-- property對應JavaBean中的屬性名 -->
            <result column="className" property="className"/>
        </collection>
    </resultMap>
    
    
    <!-- 查詢所有的老師級各自的所有學生 -->
    <select id="getTeachers" parameterType="Teacher" resultMap="teacherMap">
        SELECT
            t.id,
            t.NAME,
            t.class_Name,
            s.id AS sid,
            s. NAME AS sname,
            s.class_name as className
        FROM
            teacher t
        LEFT JOIN student s ON t.id = s.teacher_id
    </select>
</mapper>

請注意看注釋，很重要哦~

測試類：

package com.tz.test;
 
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import com.tz.mybatis.bean.Student;
import com.tz.mybatis.bean.Teacher;
 
public class TeacherTest {
 
    private SqlSessionFactory sqlSessionFactory;
    
    @Before
    public void init() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }
    
    @Test
    public void getTeachers() {
        SqlSession session = sqlSessionFactory.openSession();
        List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getTeachers");
        System.out.println(list);
    }
    
}

下面給出第二種寫法：

<!-- //////////////////////////////////////////////一對多的第二種寫法///////////////////////////////////////////////////// -->
    <resultMap type="Teacher" id="teacherMaps">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="class_name" property="className"/>
        <collection property="students" ofType="Student" select="getStudents" column="id">
        </collection>
    </resultMap>
    
    
    <!-- 查詢所有的老師級各自的所有學生 -->
    <select id="getAllTeacher" parameterType="Teacher" resultMap="teacherMaps">
        SELECT
            t.id,
            t.NAME,
            t.class_name
        FROM
            teacher t
    </select>
    
    <select id="getStudents" parameterType="int" resultType="Student">
        select 
            s.id,
            s. NAME,
            s.class_name as className
        from student s
        where teacher_id = #{id}
    </select>

測試類：

@Test
public void getTeachers2() {
    SqlSession session = sqlSessionFactory.openSession();
    List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getAllTeacher");
    System.out.println(list);
}

查詢學生信息(多對一)：

首先還是配置文件：

<resultMap type="Student" id="studentMap">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="class_name" property="className"/>
        <result column="teacher_id" property="teacherId"/>
        <association property="teacher" select="getTeacher" column="teacher_id" javaType="Teacher">
        <!-- 這里要注意的是column對應的是student中的外鍵，而且需是表字段名 -->
        </association>
    </resultMap>
    
    
    <select id="getStudent" resultMap="studentMap">
        SELECT
            s.id,
            s.name,
            s.class_name,
            s.teacher_id
        FROM
            student s
    </select>
    
    <select id="getTeacher" resultType="Teacher" parameterType="int">
        SELECT
            t.id,
            t.name,
            t.class_name as className 
        FROM teacher t 
        where id = #{teacher_id}
    </select>

測試類：

@Test
public void getStudents() {
    SqlSession session = sqlSessionFactory.openSession();
    List<Student> list = session.selectList("com.tz.mybatis.dao.studentDao.getStudent");
    System.out.println(list);
}

最后：當然如果不想配置這么麻煩的信息，可以直接寫一個關聯查詢的SQL語句，返回結果直接由Map接受即可。不過這樣就不太符合面向對象的理念了。