MyBatis关系映射--一对一关系映射

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qq5963a5404b339 2022-07-21 15:35:30 博主文章分类：mybatis ©著作权

文章标签 MyBatis关系映射--一对一映射查询映射和结果映射 MyBatis映射实例 文章分类 运维

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MyBatis关系映射--一对一关系

1.关系映射
2.关系映射的方式

2.1查询映射
2.2结果映射
2.3不同方式的分析

3.例子

3.1创建一个MyBatis工程
3.2导入jar包
3.3实现Java类
3.4properties
3.5resource
3.6mapper

4.运行结果
5.总结

1.关系映射

实际开发中，常常需要关联表进行查询，比如公民和身份证之间的联合查询，在公司，一个人有一个人的工位，一个人有一个人工作的计算机，一个工位可能有工作的计算机，等等。
一对一关系是最简单的关系，也是最基本的关系。

2.关系映射的方式

1.通过查询映射
2.通过结果映射

2.1查询映射

查询映射就是查第一个表时，取出第一个表时，查询第二个表，然后把第一个表和第二个表的结果共同返回去。查询映射就相当于查询2次，使用了2条select语句
查询映射的写法:

<!-- 嵌套查询 -->
  <resultMap type="people" id="peopleWithCard">
    <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="card"
  select="cardMapper.selectById"></association>
  </resultMap>
  <select id="selectOneByOne" parameterType="Long" resultMap="peopleWithCard">
    select <include refid="baseMapper.str_select_people"></include>
    from <include refid="baseMapper.str_table_people"></include>
    <where>
      <if test="_parameter != null and _parameter != ''">
        and id=#{_parameter}
      </if>
    </where>
  </select>

其中cardMapper.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="cardMapper">
  <select id="selectById" parameterType="Long" resultType="card">
    select <include refid="baseMapper.str_select_card"></include>
    from <include refid="baseMapper.str_table_card"></include>
    <where>
      <if test="_parameter != null and _parameter != ''">
        and id=#{_parameter}
      </if>
    </where>
  </select>
  <insert id="insertCard" parameterType="card">
    insert into <include refid="baseMapper.str_table_card"></include>(<include refid="baseMapper.str_select_card"></include>) values(#{id},#{code})
  </insert>
</mapper>

2.2结果映射

结果映射就是通过一条select语句查询出所有的结果，然后把返回的数据封装为实体。
结果映射的写法：

<!-- 嵌套结果 -->
  <sql id="str_select_people_card">
    p.id,p.name,p.age,p.sex,p.card_id,c.code
  </sql>
  <sql id="str_table_people_card">
    people p,card c
  </sql>
  <resultMap type="people" id="resultPeopleWithCard">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <result property="sex" column="sex"/>
    <association property="card" javaType="card">
      <id property="id" column="card_id"/>
      <result property="code" column="code"/>
    </association>
  </resultMap>
  <select id="selectResult" parameterType="Long" resultMap="resultPeopleWithCard">
    select <include refid="str_select_people_card"></include>
    from <include refid="str_table_people_card"></include>
    <where>
      c.id = p.card_id
      <if test="_parameter != null and _parameter != ''">
        and p.id=#{_parameter}
      </if>
    </where>
  </select>

2.3不同方式的分析

首先关系的映射在不同的方式中，使用不同的方法来实现映射。
在查询映射中，映射关系是通过一条新的通过id查询第二个表的SQL来完成的，如果没有通过id查询第二个表的映射，或者这个映射出现了问题，那么整体的查询映射也不会实现。也就是说查询映射依赖于外部已实现的通过id查询（或者其他主键查询，结果必须唯一）
在结果映射中，映射关系是通过SQL语句的where条件来实现的，mapper的映射只是实现结果的封装。结果映射不依赖与任何外部资源，而且结果映射只需要一条select即可实现。
优缺点：
查询映射，需要2条SQL实现；
结果映射，不需要依赖于外部资源；

查询映射，复用性较好，SQL难度较低；
结果映射，复用性差，SQL难度高。

特别是多个表进行关系的映射时，如果使用查询映射，其查询逻辑较为简单，易于修改，易于开发。
但是因为查询映射需要执行多条语句，肯定没有一次性返回结果的效率高，而且因为查询映射需要依赖多个外部资源，集成难度大，结果映射只需要专注于写SQL即可。

3.例子

3.1创建一个MyBatis工程

MyBatis关系映射--一对一关系映射_查询映射和结果映射

3.2导入jar包

MyBatis关系映射--一对一关系映射_MyBatis映射实例_02

3.3实现Java类

MyBatis关系映射--一对一关系映射_MyBatis关系映射--一对一映射_03

package com.client;

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

import com.util.MyBatisSessionUtils;

public class Main {
  
  @Test
  public void queryCard(){
    SqlSession session = MyBatisSessionUtils.getSession();
    System.out.println(session.selectOne("cardMapper.selectById",1));
    session.close();
  }
  
  @Test
  public void queryPeople(){
    SqlSession session = MyBatisSessionUtils.getSession();
    System.out.println(session.selectOne("peopleMapper.selectById", 103L));
    session.close();
  }
  
  @Test
  public void queryPeopleWithCard(){
    SqlSession session = MyBatisSessionUtils.getSession();
    System.out.println(session.selectOne("oneByOneMapper.selectOneByOne",103L));
    session.close();
  }
  
  @Test
  public void queryPeopleWithCardResult(){
    SqlSession session = MyBatisSessionUtils.getSession();
    System.out.println(session.selectOne("oneByOneMapper.selectResult",103L));
    session.close();
  }
  
}

package com.domain;

import java.io.Serializable;

public class Card implements Serializable{

  /**
   * 
   */
  private static final long serialVersionUID = -5398573250153908121L;

  private Long id;
  
  private String code;

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getCode() {
    return code;
  }

  public void setCode(String code) {
    this.code = code;
  }

  @Override
  public String toString() {
    return "[id="+this.id+",code="+this.code+"]";
  }
  
}

package com.domain;

import java.io.Serializable;

public class People implements Serializable{

  /**
   * 
   */
  private static final long serialVersionUID = -3270893239281340723L;

  private Long id;
  
  private String name;
  
  private Integer age;
  
  private Integer sex;
  
  private Card card;

  public Long getId() {
    return id;
  }

  public void setId(Long 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 Integer getSex() {
    return sex;
  }

  public void setSex(Integer sex) {
    this.sex = sex;
  }

  public Card getCard() {
    return card;
  }

  public void setCard(Card card) {
    this.card = card;
  }

  @Override
  public String toString() {
    return "people [id=" + this.id + ",name=" + this.name + ",age="
        + this.age + ",sex=" + this.sex + ",code=" + this.card + "]";
  }

}

package com.util;

import java.io.Reader;

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 MyBatisSessionUtils {

  private static SqlSessionFactory sessionFactory = null;
  
  private static final String PATH="com/resource/mybatis.xml";
  
  static{
    try{
      Reader reader = Resources.getResourceAsReader(PATH);
      sessionFactory = new SqlSessionFactoryBuilder().build(reader);
    } catch (Exception e){
      e.printStackTrace();
    }
  }
  
  public static SqlSession getSession(){
    return sessionFactory.openSession();
  }
}

3.4properties

log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.com=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

ojdbc.properties

driver=oracle.jdbc.driver.OracleDriver
url=jdbc:oracle:thin:@127.0.0.1:1521:oracle
username=study
password=study

3.5resource

mybatis.xml

<?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>
  <!-- 配置外在化 -->
  <properties resource="com/property/ojdbc.properties"></properties>
  <settings>
    <!-- 延迟加载全局 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 关联对象属性的延迟加载 -->
    <setting name="aggressiveLazyLoading" value="false"/>
  </settings>
  <!-- 改变运行时行为 -->
  <typeAliases>
    <!-- 配置别名 -->
    <typeAlias type="com.domain.People" alias="people"/>
    <typeAlias type="com.domain.Card" alias="card"/>
  </typeAliases>
  <!-- 环境配置 -->
  <environments default="oracle">
    <environment id="oracle">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
        <property name="driver" value="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
    <mapper resource="com/mapper/baseMapper.xml"/>
    <mapper resource="com/mapper/cardMapper.xml"/>
    <mapper resource="com/mapper/peopleMapper.xml"/>
    <mapper resource="com/mapper/oneByOneMapper.xml"/>
  </mappers>
</configuration>

3.6mapper

baseMapper.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="baseMapper">
  <sql id="str_select_people">
    p.id,p.name,p.age,p.sex,p.card_id
  </sql>
  <sql id="str_table_people">
    people p
  </sql>
  <sql id="str_select_card">
    c.id, c.code
  </sql>
  <sql id="str_table_card">
    card c
  </sql>
</mapper>

cardMapper.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="cardMapper">
  <select id="selectById" parameterType="Long" resultType="card">
    select <include refid="baseMapper.str_select_card"></include>
    from <include refid="baseMapper.str_table_card"></include>
    <where>
      <if test="_parameter != null and _parameter != ''">
        and id=#{_parameter}
      </if>
    </where>
  </select>
  <insert id="insertCard" parameterType="card">
    insert into <include refid="baseMapper.str_table_card"></include>(<include refid="baseMapper.str_select_card"></include>) values(#{id},#{code})
  </insert>
</mapper>

peopleMapper.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="peopleMapper">
  <select id="selectById" parameterType="Long" resultType="people">
    select <include refid="baseMapper.str_select_people"></include>
    from <include refid="baseMapper.str_table_people"></include>
    <where>
      <if test="_parameter != null and _parameter != ''">
        and id=#{_parameter}
      </if>
    </where>
  </select>
  <update id="updatePeopleForCard" parameterType="people">
    update <include refid="baseMapper.str_table_people"></include>
    card_id = #{card.id}
    <where>
      <if test="card_id != null and card_id != ''">
        and id=#{id}
      </if>
    </where>
  </update>
</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="oneByOneMapper">
  <!-- 嵌套查询 -->
  <resultMap type="people" id="peopleWithCard">
    <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="card"
  select="cardMapper.selectById"></association>
  </resultMap>
  <select id="selectOneByOne" parameterType="Long" resultMap="peopleWithCard">
    select <include refid="baseMapper.str_select_people"></include>
    from <include refid="baseMapper.str_table_people"></include>
    <where>
      <if test="_parameter != null and _parameter != ''">
        and id=#{_parameter}
      </if>
    </where>
  </select>
  <!-- 嵌套结果 -->
  <sql id="str_select_people_card">
    p.id,p.name,p.age,p.sex,p.card_id,c.code
  </sql>
  <sql id="str_table_people_card">
    people p,card c
  </sql>
  <resultMap type="people" id="resultPeopleWithCard">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <result property="sex" column="sex"/>
    <association property="card" javaType="card">
      <id property="id" column="card_id"/>
      <result property="code" column="code"/>
    </association>
  </resultMap>
  <select id="selectResult" parameterType="Long" resultMap="resultPeopleWithCard">
    select <include refid="str_select_people_card"></include>
    from <include refid="str_table_people_card"></include>
    <where>
      c.id = p.card_id
      <if test="_parameter != null and _parameter != ''">
        and p.id=#{_parameter}
      </if>
    </where>
  </select>
</mapper>

4.运行结果

queryCard

log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
[id=1,code=aPeopleupdateMyBatisupdateMyBatis1011]

queryPeople

log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
people [id=103,name=aPeopleupdate,age=53,sex=0,code=null]

queryPeopleWithCard

log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
people [id=103,name=aPeopleupdate,age=53,sex=0,code=[id=1,code=aPeopleupdateMyBatisupdateMyBatis1011]]

queryPeopleWithCardResult

log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
people [id=103,name=aPeopleupdate,age=53,sex=0,code=[id=1,code=aPeopleupdateMyBatisupdateMyBatis1011]]

5.总结

查询映射与结果映射这两种一对一关系的实现，根据需要进行选择，如果有已经实现且通过测试的查询，那么应该选择查询映射，如果没有实现的查询，且关联的表多个，那么尽可能使用结果映射。按需选择。

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