MyBatis一对一，一对多，多对一关联查询

1、一对一关联查询的案例

（1）需求

 根据班级id查询班级信息（带老师信息）

（2）创建表和数据

 创建一张教师表和班级表，这里我们假设一个老师只负责教一个班，那么老师和班级之间的关系就是一种一对一的关系。CREATE TABLE teacher(
t_id INT PRIMARY KEY AUTO_INCREMENT,
t_name VARCHAR(20)
);
CREATE TABLE class(
c_id INT PRIMARY KEY AUTO_INCREMENT,
c_name VARCHAR(20),
teacher_id INT
);
ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);

INSERT INTO teacher(t_name) VALUES('teacher1');
INSERT INTO teacher(t_name) VALUES('teacher2');

INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);

（3）构建实体

老师：public class Teacher {

// 定义实体类的属性，与teacher表中的字段对应
private int id; // id===>t_id
private String name; // name===>t_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;
}

@Ov
4000
erride
public String toString() {
return "Teacher [id=" + id + ", name=" + name + "]";
}
} 班级：public class Classes {

// 定义实体类的属性，与class表中的字段对应
private int id; // id===>c_id
private String name; // name===>c_name

/**
* class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
* 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
*/
private Teacher teacher;

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 Teacher getTeacher() {
return teacher;
}

public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}

@Override
public String toString() {
return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
+ "]";
}
}

（4）定义Mapper对象

public interface ClassMapper {
/**
* 用于测试，非懒加载的one2one的实现
*
* @param id
* @return
*/
public Classes getClass1(int id);

/**
* 用于测试，懒加载的one2one的实现
*
* @param id
* @return
*/
public Classes getClass2(int id);
}

（5）定义sql映射文件classMapper.xml<?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.baowei.mapper.ClassMapper">

<!--
根据班级id查询班级信息(带老师的信息)
##1. 联表查询(非懒加载的实现)
SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;

##2. 执行两次查询(懒加载的实现)
SELECT * FROM class WHERE c_id=1; //teacher_id=1
SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
-->

<!--
方式一：(非懒加载的实现)
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
封装联表查询的数据(去除重复的数据)
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
-->
<select id="getClass1" parameterType="int" resultMap="ClassResultMap">
select *
from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="com.baowei.entity.Classes" id="ClassResultMap">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" javaType="com.baowei.entity.Teacher">
<id property="id" column="t_id" />
<result property="name" column="t_name" />
</association>
</resultMap>

<!--
方式二：(懒加载的实现)
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;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
-->
<select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select *
from class where c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="com.baowei.entity.Classes" id="ClassResultMap2">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id"
select="getTeacher" />
</resultMap>
<select id="getTeacher" parameterType="int"
resultType="com.baowei.entity.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>

</mapper>

(6)SqlMapConfig.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

<!--用于加载数据库的连接配置 -->
<properties resource="db.properties" />
<!--用于设置懒加载的设置 -->
<settings>
<!-- 打开延迟加载的开关 -->
<setting name="lazyLoadingEnabled" value="true" />
<!-- 将积极加载改为消息加载即按需加载 -->
<setting name="aggressiveLazyLoading" value="false" />
</settings>
<!--数据库的连接配置 -->
<environments default="development">
<environment id="development">
<!-- 使用jdbc事务管理，事务控制由mybatis -->
<transactionManager type="JDBC" />
<!-- 数据库连接池，由mybatis管理 -->
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</dataSource>
</environment>
</environments>
<!-- 加载 映射文件 -->
<mappers>
<package name="com.baowei.mapper" />
</mappers>

</configuration>

（7）.MyBatisUtil.java工具

import java.io.IOException;
import java.io.InputStream;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class MyBatisUtil {

/**
* 获取SqlSessionFactory
*
* @return SqlSessionFactory
*/
public static SqlSessionFactory getSqlSessionFactory() {
String resource = "SqlMapConfig.xml";
InputStream inputStream = null;
SqlSessionFactory sqlSessionFactory = null;
try {
inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder()
.build(inputStream);

} catch (IOException e) {
e.printStackTrace();
}
return sqlSessionFactory;
}

/**
* 获取SqlSession
*
* @return SqlSession
*/
public static SqlSession getSqlSession() {
return getSqlSessionFactory().openSession();
}

/**
* 获取SqlSession
*
* @param isAutoCommit
*            true 表示创建的SqlSession对象在执行完SQL之后会自动提交事务 false
*            表示创建的SqlSession对象在执行完SQL之后不会自动提交事务
*            ，这时就需要我们手动调用sqlSession.commit()提交事务
* @return SqlSession
*/
public static SqlSession getSqlSession(boolean isAutoCommit) {
return getSqlSessionFactory().openSession(isAutoCommit);
}

public static void main(String[] args) {
System.out.println(getSqlSessionFactory());
}
}

（8）测试代码

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

import com.baowei.entity.Classes;
import com.baowei.mapper.ClassMapper;
import com.baowei.utils.MyBatisUtil;

public class TestOne2One {

/**
* 非懒加载的one2one的测试
*/
@Test
public void testGetClass() {
SqlSession sqlSession = MyBatisUtil.getSqlSession();

ClassMapper mapper = sqlSession.getMapper(ClassMapper.class);
Classes clazz = mapper.getClass1(1);

// 使用SqlSession执行完SQL之后需要关闭SqlSession
sqlSession.close();
System.out.println(clazz.getId());
}

/**
* 懒加载的one2one的测试
*/
@Test
public void testGetClass2() {
SqlSession sqlSession = MyBatisUtil.getSqlSession();

ClassMapper mapper = sqlSession.getMapper(ClassMapper.class);
Classes clazz = mapper.getClass2(1);

// 使用SqlSession执行完SQL之后需要关闭SqlSession
sqlSession.close();
System.out.println(clazz.getId());
// 可用于测试懒加载(查看日志的sql输出结果，就可以发现使用了懒加载)
//System.out.println(clazz.getTeacher());

}
}

2.一对多/多对一查询案例

（1）需求

本实例使用顾客和订单的例子做说明: 一个顾客可以有多个订单, 一个订单只对应一个顾客。

（2）创建表和数据

CREATE DATABASE test;

USE test;

CREATE TABLE person(
personId VARCHAR(36) PRIMARY KEY,
personName VARCHAR(64),
personAddress VARCHAR(128),
personTel VARCHAR(11)
);

CREATE TABLE orders(
orderId VARCHAR(36) PRIMARY KEY,
orderNumber VARCHAR(20),
orderPrice INT,
pid VARCHAR(36)
);

INSERT INTO person VALUES('001', 'Jack', 'Wuhan', '1234567');
INSERT INTO orders VALUES('O_00001', '00001', 100, '001');
INSERT INTO orders VALUES('O_00002', '00002', 200, '001');

SELECT p.*, o.*
FROM person p
JOIN orders o ON (p.personId=o.pid)
WHERE p.personId = '001'

（3）构建实体

客户：public class Person {

private String id;
private String name;
private String address;
private String tel;

private List<Order> orders;

@Override
public String toString() {
return "{id: " + id + ", name: " + name + ", address: " + address + ", tel: " + tel + "}";
}
} 订单：public class Order {

private String id;
private String number;
private int price;

private Person person;

@Override
public String toString() {
return "{id: " + id + ", number: " + number + ", price: " + price + "}";
}

}

（4）一对多实体配置: Person.xml

<?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.zdp.domain.Person">

<resultMap type="Person" id="personBean">
<id column="personId" property="id"/>
<result column="personName" property="name"/>
<result column="personAddress" property="address"/>
<result column="personTel" property="tel"/>

<!-- 一对多的关系 -->
<!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->
<collection property="orders" ofType="Order">
<id column="orderId" property="id"/>
<result column="orderNumber" property="number"/>
<result column="orderPrice" property="price"/>
</collection>
</resultMap>

<!-- 根据id查询Person, 关联将Orders查询出来 -->
<select id="selectPersonById" parameterType="string" resultMap="personBean">
select p.*, o.* from person p, orders o where p.personId  = o.pid and p.personId = #{id}
</select>

</mapper>

 （5）多对一实体配置:order.xml

<?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.zdp.domain.Order">
<resultMap type="Order" id="orderBean">
<id column="orderId" property="id"/>
<result column="orderNumber" property="number"/>
<result column="orderPrice" property="price"/>

<!-- 多对一的关系 -->
<!-- property: 指的是属性的值, javaType：指的是属性的类型-->
<association property="person" javaType="Person">
<id column="personId" property="id"/>
<result column="personName" property="name"/>
<result column="personAddress" property="address"/>
<result column="personTel" property="tel"/>
</association>
</resultMap>

<!-- 根据id查询Order, 关联将Person查询出来 -->
<select id="selectOrderById" parameterType="string" resultMap="orderBean">
select p.*, o.* from person p, orders o where p.personId  = o.pid and o.orderId = #{id}
</select>

</mapper>

(6)SqlMapConfig.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<typeAliases>
<typeAlias type="com.zdp.domain.Person" alias="Person"/>
<typeAlias type="com.zdp.domain.Order" alias="Order"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost/test" />

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<property name="username" value="root" />
<property name="password" value="root" />
</dataSource>
</environment>
</environments>
<mappers>
<!-- 映射文件的位置 -->
<mapper resource="com/zdp/domain/Person.xml" />
<mapper resource="com/zdp/domain/Order.xml" />
</mappers>
</configuration>

(7)测试代码

/**
* 测试一对多和多对一
*/
public class MybatisTest {

private SqlSessionFactory ssf;

@Before
public void initSF() throws Exception {
String resource = "sqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
ssf = new SqlSessionFactoryBuilder().build(inputStream);
}

@Test//一对多关联查询
public void selectPersonById()throws Exception{
SqlSession session = ssf.openSession();
Person person = session.selectOne("com.zdp.domain.Person.selectPersonById", "001");
System.out.println(person.getOrders());
}

@Test//多对一关联查询
public void selectOrderById()throws Exception{
SqlSession session = ssf.openSession();
Order order = session.selectOne("com.zdp.domain.Order.selectOrderById", "O_00001");
System.out.println(order.getPerson().getName());
}
}

3.参考博文

(1)http://www.cnblogs.com/xdp-gacl/p/4264440.html
(2)http://blog.csdn.net/jkxiaoxing/article/details/52199386