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<Student> 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。
<?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">
<!--
4、关联表查询：一对多关联
如何根据class_id查询班级信息（包括老师和学生信息）,学生信息为集合List
Class封装了Teacher和学生属性，即一张class表中包含teacher表和student表
-->
<!--定义操作 classes 表的sql 映射文件：classesMapper.xml -->
<mapper namespace="com.mybatis.test5.classesMapper2">
<!--
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
-->
<select id="getClassInfo3" parameterType="int"
resultMap="getClassMap">
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>
<!-- 解决字段名和属性不一致冲突 -->
<resultMap type="Classes" id="getClassMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<!-- 关联的教师信息 -->
<association property="teacher" column="teacher_id"
javaType="Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<!-- 关联的学生信息，是集合 -->
<collection property="students"
ofType="Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</collection>
</resultMap>
<!--
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
-->
<select id="getClassInfo4" parameterType="int" resultMap="getClassMap2">
SELECT * FROM class WHERE c_id =#{id}
</select>
<resultMap type="Classes" id="getClassMap2">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="Teacher"
select="getTeacher"></association>
<collection property="students" column="c_id" ofType="Student"
select="getStudent"></collection>
</resultMap>
<select id="getTeacher" resultType="Teacher">
SELECT t_id id,t_name name FROM teacher WHERE t_id =#{id}
</select>
<select id="getStudent" resultType="Student">
SELECT s_id id,s_name name FROM student WHERE class_id =#{id}
</select>
</mapper>
4 注册：conf.xml
<mappers>
<mapper resource="com/mybatis/test5/classesMapper2.xml"/>
</mappers>
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();
}
}
