Mybatis关联查询之一对多和多对一XML配置详解

平时在开发过程中dao、bean和XML文件都是自动生成的，很少写XML的配置关系，今天记录一下mybatis的关联查询中的多对一和一对多的情况。

• 首先是有两张表(学生表Student和老师Teacher表，注：这里只是为了演示一对多和多对一的情况，请不要杠)，为了更易懂，这里只设置了最简单的几个必要字段。表结构如下图

Student表：

Teacher表：

• 创建实体bean

Teacher.java:

import java.util.List;

public class Teacher {

	private Integer id;
	private String name;
	private String className;
	private List students;

	// get、set方法省略

}

Sfudent.java

public class Student {

	private Integer id;
	private String name;
	private Integer teacherId;
	private String className;
	private Teacher teacher;
	
    // get、set方法省略
} 

• 下面重点来了：配置Mapper.xml文件：

  
	
	
	
	    
	    
            
                
		    
                
		    
		    
		    
	
	
	
	
	
		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
	

• 测试类：

 

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 list = session.selectList("com.tz.mybatis.dao.studentDao.getTeachers");
		System.out.println(list);
	}
	
}

下面给出第二种写法：

	
		
		
		
		
		
	
	
	
	
	
		SELECT
			t.id,
			t.NAME,
			t.class_name
		FROM
			teacher t
	
	
	
		select 
			s.id,
			s. NAME,
			s.class_name as className
		from student s
		where teacher_id = #{id}
	

测试类：

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

查询学生信息(多对一)：

首先还是配置文件：

		
		
		
		
		
		
		
	
	
	
	
		SELECT
			s.id,
			s.name,
			s.class_name,
			s.teacher_id
		FROM
			student s
	
	
	
		SELECT
			t.id,
			t.name,
			t.class_name as className 
		FROM teacher t 
		where id = #{teacher_id}
	

测试类：

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

最后：当然如果不想配置这么麻烦的信息，可以直接写一个关联查询的SQL语句，返回结果直接由Map接受即可。不过这样就不太符合面向对象的理念了。