Mybatis怎么实现一对一、一对多关联查询

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Mybatis实现一对一、一对多关联查询，关联查询：多个表联合查询，只查询一次，通过resultMap里面的<association>、<collection>标签配置一对一、一对多。

涉及的sql文件、idea工程文件：Mybatis实现一对一、一对多关联查询资源

• 班级classes、班主任teacher是一对一的关系

• 班级classes、学生student是一对多的关系

使用IDEA快速构建一个MyBatis工程

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com</groupId>
    <artifactId>sun</artifactId>
    <version>1.0-SNAPSHOT</version>
    <name>mybatisTest</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.target>1.8</maven.compiler.target>
        <maven.compiler.source>1.8</maven.compiler.source>
        <junit.version>5.8.2</junit.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.2.2</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.30</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.24</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>2.13.3</version>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>${junit.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>${junit.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13.2</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
    <build>
        <resources>
            <resource>
                <!-- directory：指定资源文件的位置 -->
                <directory>src/main/java</directory>
                <includes>
                    <!-- “**” 表示任意级目录    “*”表示任意任意文件 -->
                    <!-- mvn resources:resources　：对资源做出处理，先于compile阶段  -->
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <!--  filtering：开启过滤，用指定的参数替换directory下的文件中的参数(eg. ${name}) -->
                <filtering>false</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
            </resource>
        </resources>
    </build>
</project>

mybatis-config.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">

<!-- 通过这个配置文件完成mybatis与数据库的连接 -->
<configuration>
    <!-- 引入 database.properties 文件-->
    <properties resource="database.properties"/>

    <!-- 配置mybatis的log实现为LOG4J -->
    <settings>
        <setting name="logImpl" value="LOG4J" />
    </settings>

    <environments default="development">
        <environment id="development">
            <!--配置事务管理，采用JDBC的事务管理  -->
            <transactionManager type="JDBC"></transactionManager>
            <!-- POOLED:mybatis自带的数据源，JNDI:基于tomcat的数据源 -->
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${user}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 将mapper文件加入到配置文件中 -->
    <mappers>
        <mapper resource="com/sun/mapper/ClassesMapper.xml"/>
    </mappers>
</configuration>

database.properties

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/mybatis?useUnicode=true&characterEncoding=utf-8&serverTimezone=Asia/Shanghai
user=root
password=root

MybatisUtil.java

package com.sun.utils;

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

import java.io.InputStream;

/**
 * @author JumperSun
 * @date 2023-03-26-14:46
 */
public class MyBatisUtil {
    private static SqlSessionFactory factory;

    static {
        try {
            InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
            factory = new SqlSessionFactoryBuilder().build(is);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static SqlSession createSqlSession(){
        return factory.openSession(false); // true为自动提交事务
    }

    public static void closeSqlSession(SqlSession sqlSession){
        if(null != sqlSession)
            sqlSession.close();
    }
}

Teacher.java

package com.sun.pojo;

import lombok.Data;

/**
 * @author JumperSun
 * @date 2023-03-26-14:51
 */
@Data
public class Teacher {
    private int id;
    private String name;
}

Student.java

package com.sun.pojo;

import lombok.Data;

/**
 * @author JumperSun
 * @date 2023-03-26-14:53
 */
@Data
public class Student {
    private int id;
    private String name;
}

Classes.java

package com.sun.pojo;

import lombok.Data;

import java.util.List;

/**
 * @author JumperSun
 * @date 2023-03-26-14:52
 */
@Data
public class Classes {
    private int id;
    private String name;
    private Teacher teacher;
    private List<Student> studentList;
}

classMapper.java

package com.sun.mapper;

import com.sun.pojo.Classes;
import org.apache.ibatis.annotations.Param;

import java.util.List;

/**
 * @author JumperSun
 * @date 2023-03-26-14:54
 */
public interface ClassesMapper {
    List<Classes> getClassesTeacher(@Param("id") int id);

    List<Classes> getClassesTeacherStudentList(@Param("id") int id);
}

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.sun.mapper.ClassesMapper">

    <!-- 一对一关联查询 -->
    <select id="getClassesTeacher" parameterType="int" resultMap="ClassesResultMap">
        select * from classes c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
    </select>

    <resultMap type="com.sun.pojo.Classes" id="ClassesResultMap">
        <!-- 实体类的字段名和数据表的字段名映射 -->
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="com.sun.pojo.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>

    <!-- 一对多关联查询 -->
    <select id="getClassesTeacherStudentList" parameterType="int" resultMap="ClassesResultMap2">
        select * from classes 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>

    <resultMap type="com.sun.pojo.Classes" id="ClassesResultMap2">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="com.sun.pojo.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
        <collection property="studentList" ofType="com.sun.pojo.Student">
            <id property="id" column="s_id"/>
            <result property="name" column="s_name"/>
        </collection>
    </resultMap>

</mapper>

MapperTest.java

import com.sun.mapper.ClassesMapper;
import com.sun.pojo.Classes;
import com.sun.utils.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

/**
 * @author JumperSun
 * @date 2023-03-26-15:10
 */
public class MapperTest {
    /**
     * 测试班级与班主任老师一对一的关联关系
     */
    @Test
    public void test1() {
        SqlSession sqlSession = MyBatisUtil.createSqlSession();
        ClassesMapper classesMapper = sqlSession.getMapper(ClassesMapper.class);
        List<Classes> classes = classesMapper.getClassesTeacher(1);
        for (Classes Classes : classes) {
            System.out.println(classes);
        }

    }

    /**
     * 测试班级与学生一对多关联关系
     */
    @Test
    public void test2() {
        SqlSession sqlSession = MyBatisUtil.createSqlSession();
        ClassesMapper classesMapper = sqlSession.getMapper(ClassesMapper.class);
        List<Classes> classes = classesMapper.getClassesTeacherStudentList(1);
        for (Classes Classes : classes) {
            System.out.println(classes);
        }

    }
}

test1:

test2:

总结：一对一关联查询需要在resultMap里面配置association，一对多关联查询需要在resultMap里面配置collection；

List classes = classesMapper.getClassesTeacherStudentList(1);
for (Classes Classes : classes) {
System.out.println(classes);
}}

}
test1:
[外链图片转存中...(img-qon5qYiR-1680248446105)]
test2:
[外链图片转存中...(img-DZayGx8q-1680248446107)]
总结：一对一关联查询需要在resultMap里面配置association，一对多关联查询需要在resultMap里面配置collection；

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