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MyBatis——实现关联表查询

原文:http://www.cnblogs.com/xdp-gacl/p/4264440.html

一、一对一关联

 1.1、提出需求

  根据班级id查询班级信息(带老师的信息)

1.2、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);

  技术分享

1.3、定义实体类

  1、Teacher类,Teacher类是teacher表对应的实体类。

package me.gacl.domain;

/**
 * @author gacl
 * 定义teacher表对应的实体类
 */
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;
    }

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

2、Classes类,Classes类是class表对应的实体类

package me.gacl.domain;

/**
 * @author gacl
 * 定义class表对应的实体类
 */
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+ "]";
    }
}

1.4、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">
<!-- 为这个mapper指定一个唯一的namespace,namespace的值习惯上设置成包名+sql映射文件名,这样就能够保证namespace的值是唯一的
例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
 -->
<mapper namespace="me.gacl.mapping.classMapper">

    <!-- 
        根据班级id查询班级信息(带老师的信息)
        ##1. 联表查询
        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
        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: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="getClass" 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="me.gacl.domain.Classes" id="ClassResultMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="me.gacl.domain.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </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;
        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="me.gacl.domain.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="me.gacl.domain.Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
     </select>

</mapper>

在conf.xml文件中注册classMapper.xml

<mappers>
        <!-- 注册classMapper.xml文件, 
        classMapper.xml位于me.gacl.mapping这个包下,所以resource写成me/gacl/mapping/classMapper.xml-->
        <mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers>

1.5、编写单元测试代码

package me.gacl.test;

import me.gacl.domain.Classes;
import me.gacl.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class Test3 {
    
    @Test
    public void testGetClass(){
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        /**
         * 映射sql的标识字符串,
         * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值,
         * getClass是select标签的id属性值,通过select标签的id属性值就可以找到要执行的SQL
         */
        String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
        //执行查询操作,将查询结果自动封装成Classes对象返回
        Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
        //使用SqlSession执行完SQL之后需要关闭SqlSession
        sqlSession.close();
        System.out.println(clazz);//打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
    }
    
    @Test
    public void testGetClass2(){
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        /**
         * 映射sql的标识字符串,
         * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值,
         * getClass2是select标签的id属性值,通过select标签的id属性值就可以找到要执行的SQL
         */
        String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的标识字符串
        //执行查询操作,将查询结果自动封装成Classes对象返回
        Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
        //使用SqlSession执行完SQL之后需要关闭SqlSession
        sqlSession.close();
        System.out.println(clazz);//打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
    }
}
按 Ctrl+C 复制代码

 1.6、MyBatis一对一关联查询总结

  MyBatis中使用association标签来解决一对一的关联查询,association标签可用的属性如下:

  • property:对象属性的名称
  • javaType:对象属性的类型
  • column:所对应的外键字段名称
  • select:使用另一个查询封装的结果

二、一对多关联

2.1、提出需求

  根据classId查询对应的班级信息,包括学生,老师

2.2、创建表和数据

  在上面的一对一关联查询演示中,我们已经创建了班级表和教师表,因此这里再创建一张学生表

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(student_A, 1);
INSERT INTO student(s_name, class_id) VALUES(student_B, 1);
INSERT INTO student(s_name, class_id) VALUES(student_C, 1);
INSERT INTO student(s_name, class_id) VALUES(student_D, 2);
INSERT INTO student(s_name, class_id) VALUES(student_E, 2);
INSERT INTO student(s_name, class_id) VALUES(student_F, 2);

技术分享

2.3、定义实体类

  1、Student类

package me.gacl.domain;

/**
 * @author gacl
 * 定义student表所对应的实体类
 */
public class Student {

    //定义属性,和student表中的字段对应
    private int id;            //id===>s_id
    private String name;    //name===>s_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;
    }

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

2、修改Classes类,添加一个List<Student> students属性,使用一个List<Student>集合属性表示班级拥有的学生,如下:

package me.gacl.domain;

import java.util.List;

/**
 * @author gacl
 * 定义class表对应的实体类
 */
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;
    //使用一个List<Student>集合属性表示班级拥有的学生
    private List<Student> students;

    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;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

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

2.4、修改sql映射文件classMapper.xml

  添加如下的SQL映射信息

<!-- 
        根据classId查询对应的班级信息,包括学生,老师
     -->
    <!-- 
    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="getClass3" parameterType="int" resultMap="ClassResultMap3">
        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>
    <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
        <!-- ofType指定students集合中的对象类型 -->
        <collection property="students" ofType="me.gacl.domain.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;
            SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值
            SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值
     -->
     <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
        select * from class where c_id=#{id}
     </select>
     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
        <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
     </resultMap>
     
     <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
     </select>
     
     <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
        SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
     </select>

2.5、编写单元测试代码

package me.gacl.test;

import me.gacl.domain.Classes;
import me.gacl.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class Test4 {
    
    @Test
    public void testGetClass3(){
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        /**
         * 映射sql的标识字符串,
         * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值,
         * getClass3是select标签的id属性值,通过select标签的id属性值就可以找到要执行的SQL
         */
        String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串
        //执行查询操作,将查询结果自动封装成Classes对象返回
        Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
        //使用SqlSession执行完SQL之后需要关闭SqlSession
        sqlSession.close();
        //打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
        System.out.println(clazz);
    }
    
    @Test
    public void testGetClass4(){
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        /**
         * 映射sql的标识字符串,
         * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值,
         * getClass4是select标签的id属性值,通过select标签的id属性值就可以找到要执行的SQL
         */
        String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串
        //执行查询操作,将查询结果自动封装成Classes对象返回
        Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
        //使用SqlSession执行完SQL之后需要关闭SqlSession
        sqlSession.close();
        //打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
        System.out.println(clazz);
    }
}

2.6、MyBatis一对多关联查询总结

  MyBatis中使用collection标签来解决一对多的关联查询,ofType属性指定集合中元素的对象类型。

MyBatis——实现关联表查询