JavaEE MyBatis关联映射之一对一（教材学习笔记）

关联映射主要分为一对一、一对多、多对多，下面学习一对一

在学习resultMap元素时，包含了一个association子元素，这个元素就是用来处理一对一关联关系的

下面通过一个案例来学习一下

1.建立数据表

create table tb_idcard(
    id int primary key auto_increment,
    code varchar(18)
);
insert into tb_idcard(code)
values('152221198711020624'),('152221198804058713');

create table tb_person(
    id int primary key auto_increment,
    name varchar(32),
    age int,
    sex varchar(8),
    card_id int unique,
    foreign key(card_id) references tb_idcard(id)
);

insert into tb_person(name,age,sex,card_id)
values('rose',29,'女',1);
insert into tb_person(name,age,sex,card_id)
values('tom',25,'男',2);

2.创建项目chapter09，并将所需jar包以及MybatisUtils工具类以及核心配置文件mybatis—config.xml赋值粘贴过来

package com.itheima.po;

public class IdCard {

	private Integer id;
	private String code;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getCode() {
		return code;
	}
	public void setCode(String code) {
		this.code = code;
	}
	@Override
	public String toString() {
		return "IdCard [id="+id+",code="+code+"]";
	}
}

package com.itheima.po;

public class Person {
	private Integer id;
	private String name;
	private Integer age;
	private String sex;
	private IdCard card;
	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 getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	public String getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	public IdCard getCard() {
		return card;
	}
	public void setCard(IdCard card) {
		this.card = card;
	}
	@Override
	public String toString() {
		return "Customer [id="+id+",name="+name+",age="+age+",sex="+sex+",Card="+card+"]";
	}

}

3.在com.itheima.mapper包中，创建证件映射文件IdCardMapper.xml和PersonMapper.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.itheima.mapper.IdCardMapper">
    
    <select id="findCodeById" parameterType="Integer" resultType="IdCard">
        select * from tb_idcard where id=#{id};
    </select>
</mapper>

<?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.itheima.mapper.PersonMapper">
    <select id="findPersonById" parameterType="Integer" resultMap="IdCardWithPersonResult">
        select * from tb_person where id=#{id}
    </select>
    <resultMap type="person" id="IdCardWithPersonResult">
      <id property="id" column="id"/>
      <result property="name" column="name"/>
      <result property="age" column="age"/> 
      <result property="sex" column="sex"/>  
      <association property="card" column="card_id" javaType="IdCard"
          select="com.itheima.mapper.IdCardMapper.findCodeById"></association>  
    </resultMap>
</mapper>

4.在核心配置文件mybatis-config.xml文件中，引入Mapper映射文件并定义别名，

<?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>

    <properties resource="db.properties"/>
    <typeAliases>
        <package name="com.itheima.po"/>
    </typeAliases>
    <!-- 1.配置环境，默认的环境id为mysql -->
    <environments default="mysql">
        <!-- 1.2.配置id为mysql的数据库环境 -->
        <environment id="mysql">
            <!-- 使用数据库事务管理 -->
            <transactionManager type="JDBC"/>
            <!-- 数据库连接池 -->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="itcst"/>
            </dataSource>
        </environment>
    </environments>
    <!-- 2.配置Mapper的位置 -->
    <mappers>
        <mapper resource="com/itheima/mapper/IdCardMapper.xml"/>
        <mapper resource="com/itheima/mapper/PersonMapper.xml"/>
    </mappers>
</configuration>

5.创建测试类MyBatisssociatedTest，并在类中编写测试方法findPersonByIdTest（）

package com.itheima.test;

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

import com.itheima.po.Person;
import com.itheima.utils.MybatisUtils;

public class MybatisAssociatedTest {
	
	@Test
	public void findPersonByIdTest() {
		SqlSession session = MybatisUtils.getSession();
		Person person = session.selectOne("com.itheima.mapper.PersonMapper.findPersonById",1);
		System.out.println(person);
		//关闭SqlSession
	    session.close();
	}
	
	
}

6.查看测试结果

显示完全成功了，并且成功的打印了两句查询语句

7.嵌套结果方式

以上的六步都是在演示使用嵌套查询的一对一关联方法，但是在复杂的查询条件下，嵌套查询可能会很复杂甚至很缓慢，这个时候就需要用到嵌套结果方式查询。

其步骤与上面大致相同，不同之处在于PersonMapper.xml中的有关代码，修改之后的代码如下：

<select id="findPersonById2" parameterType="Integer" resultMap="IdCardWithPersonResult2">
        select p.*,idcard.code
        from tb_person p,tb_idcard idcard
        where p.card_id=idcard.id
        and idcard.id=#{id}
    </select>
    <resultMap type="person" id="IdCardWithPersonResult2">
      <id property="id" column="id"/>
      <result property="name" column="name"/>
      <result property="age" column="age"/> 
      <result property="sex" column="sex"/>  
      <association property="card" javaType="IdCard">
          <id property="id" column="card_id"/>
          <result property="code" column="code"/>
      </association>  
    </resultMap>

并创建测试方法

@Test
	public void findPersonByIdTest2() {
		SqlSession session = MybatisUtils.getSession();
		Person person = session.selectOne("com.itheima.mapper.PersonMapper.findPersonById2",1);
		System.out.println(person);
		//关闭SqlSession
	    session.close();
	}

测试结果如下：

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可以看到只打印了一条查询语句，但是查询结果和嵌套查询的查询结果是一致的