平時在開發過程中dao、bean和XML檔案都是自動生成的，很少寫XML的配置關係，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，這裡只設置了最簡單的幾個必要欄位。表結構如下圖

Student表：

Teacher表：

• 建立實體bean

Teacher.java:

import java.util.List;

/**
 * TODO
 * @version 建立時間：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;
	}

}
 

Sfudent.java

/**
 * TODO
 * 
 * @author 作者 E-mail:[email protected]
 * @version 建立時間：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檔案：

<?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，一般考慮到效能問題，不會這麼實現//////////////////////// -->
	<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"/><!-- 這裡的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>
 

• 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);
	}

查詢學生資訊(多對一)：

首先還是配置檔案：

<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">
		<!-- 這裡要注意的是column對應的是student中的外來鍵，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);
	}

最後：當然如果不想配置這麼麻煩的資訊，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物件的理念了。