【Mybatis学习总结五】实现关联表查询----一对多关联(collection)

实现关联表查询----一对多关联(collection)

一对多需求：即一张表class中又含有多张表(teacher,student)内容。现根据class_id 来获取对应的班级信息(包括学生和老师信息)。

1 、创建表和数据：

CREATE TABLE student(
s_id INT PRIMARY KEY AUTO_INCREMENT,
s_name VARCHAR(20),
class_id INT
);
INSERT INTO student(s_name, class_id) VALUES('xs_A', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_B', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_C', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_D', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_E', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

2、创建实体类：Student.java

package com.mybatis.entities;

public class Student {

	private int id;
	private String name;
	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 Student(int id, String name) {
		super();
		this.id = id;
		this.name = name;
	}
	public Student() {
		super();
	}
	@Override
	public String toString() {
		return "Student [id=" + id + ", name=" + name + "]";
	}
	
}

Classes.java 中添加属性 ：

private List students;

3、定义sql映射文件：classesMapper2.xml

方式一：嵌套结果
使用嵌套结果映射来处理重复的联合结果的子集

SELECT * FROM class c, teacher t,student s 
WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1

方式二：嵌套查询
通过执行另外一个SQL映射语句来返回预期的复杂类型

SELECT * FROM class WHERE c_id =1 //查询后获取到teacher_id,c_id值，传入下两条语句
SELECT * FROM teacher WHERE t_id =1   //t_id=1 是上条查询得到的teacher_id值
SELECT * FROM student WHERE class_id =1   //c_id = 1 是上条查询得到的c_id值

即通过三条语句分别来查询。

	
	
			SELECT * FROM class c, teacher t,student s 
			WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND  c.c_id=#{id}
	
	
	
		
		
		
			
		
			
			
		
			
		
			
				
		
	
	
	
	
			SELECT * FROM class WHERE c_id =#{id}	
	
		
		
			
			
			
			
		
	
	
			SELECT t_id id,t_name name FROM teacher WHERE t_id =#{id}
	
	
	
			SELECT s_id id,s_name name FROM student WHERE class_id =#{id}                                                                                   
	

4 注册：conf.xml

5   测试类Test5.java

package com.mybatis.test5;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.mybatis.entities.Classes;

public class Test5 {

	@Test
	public void test() {

		SqlSession session= MybatisUtils.getSqlSessionFactory().openSession();
		//方式一测试
		String statement = "com.mybatis.test5.classesMapper2.getClassInfo3";
		Classes clazz=session.selectOne(statement , 1);
		
		//方式二测试
		statement = "com.mybatis.test5.classesMapper2.getClassInfo4";
		clazz=session.selectOne(statement , 1);
		System.out.println("*"+clazz);
		session.close();
	}

}