mybatis学习（五）----实现关联表查询

一.一对一的表查询

查询班级表中班级号为1的对应的记录（包括教师的具体信息）

1.首先建立数据表

数据表class和techear，class表中只有一个外键techear_id，sql脚本如下：

 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);

建立好的数据表如下：

class表

techear表

2.实体类

Classes类对应class表，Teacher类对应teacher表

 package me.gacl.domain;

 import me.gacl.domain.Teacher;
 /**
  * 班级实体类
  */
 public class Classes {
     private int id;//id对应c_id
     private String name;//name对应c_name
     private Teacher teacher;//因为class表中只有一个teacher_id外键，所以Class类中持有Teacher类的一个对象

     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 +
                 '}';
     }
 }

 package me.gacl.domain;

 /**
  * 教师实体类
  */
 public class Teacher {
     private int id;//id对应t_id
     private String name;//name对应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 + '\'' +
                 '}';
     }
 }

3.编写sql映射文件

teacher_classMapper.xml文件如下：

 <?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">
 <!--namespace一般命名成报名+映射名-->
 <mapper namespace="me.gacl.mapping.teacher_classMapper">
     <!--查询班级为1的班级和教师信息-->
     <!--方法一：联表查询-->
     <select id="getClass_Teacher" parameterType="int" resultMap="classResultMap">
         select * from class,teacher where class.teacher_id=teacher.t_id and c_id=#{id}
     </select>
     <!--使用resultMap指定实体类字段与数据表属性之间的一一对应关系-->
     <resultMap id="classResultMap" type="Classes">
         <id column="c_id" property="id"></id>
         <result column="c_name" property="name"></result>
         <!--使用association中的javaType指定关联表中对应的实体类的类型，
         及实体类字段与数据表属性之间的一一对应关系-->
         <association property="teacher" javaType="Teacher">
             <id column="t_id" property="id"></id>
             <result column="t_name" property="name"></result>
         </association>
     </resultMap>
     <!--方法二，执行两步查询，引用另一步查询的查询结果-->
     <!--首先查询出班级为1对应的教师信息-->
     <select id="getClass_Teacher2" parameterType="int" resultMap="classResultMap2">
         select * from class where c_id=#{id}
     </select>
     <resultMap id="classResultMap2" type="Classes">
         <id column="c_id" property="id"></id>
         <result column="c_name" property="name"></result>
         <!--这里的column="teacher_id"一定不能少，因为teacher_id的值是作为参数传递给要引用的Select语句的
         相当于mybatis执行resultSet.getInt("teacher_id")，where后面的id入参就是得到的teacher_id的值-->
         <association property="teacher" column="teacher_id" select="getTeacher"></association>
     </resultMap>
     <!--然后根据上一步查询出的教师id到教师表中查询教师信息-->
     <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>

注意：引用类都是用的别名，即类名

4.注册sql映射文件

在mybatis_config.xml中注册teacher_classMapper.xml映射文件

    <mappers>
         <mapper resource="mapping/teacher_classMapper.xml"></mapper>
     </mappers>

5.编写测试类

 package me.gacl.domain;

 import me.gacl.Util.MyBatisUtil;
 import org.apache.ibatis.session.SqlSession;
 import java.util.List;
 /**
  * 联表查询的测试类
  */
 public class Class_TeacherTest {
     public void testgetClass_teacher(){
         SqlSession sqlSession = MyBatisUtil.getSqlSession(true);
         String statement = "me.gacl.mapping.teacher_classMapper.getClass_Teacher";
         Classes classes = sqlSession.selectOne(statement,1);
         sqlSession.close();
         System.out.println(classes);
     }
     public void testgetClass_teacher2(){
         SqlSession sqlSession = MyBatisUtil.getSqlSession(true);
         String statement = "me.gacl.mapping.teacher_classMapper.getClass_Teacher2";
         List <Classes> list = sqlSession.selectList(statement,1);
         sqlSession.close();
         System.out.println(list);
     }

     public static void main(String[] args) {
         Class_TeacherTest class_teacherTest = new Class_TeacherTest();
         //class_teacherTest.testgetClass_teacher();
         class_teacherTest.testgetClass_teacher2();
     }
 }

测试结果：两个方法输出结果一样

6.总结

mybatis中实现一对一的多表查询，使用association标签实现，association标签的属性介绍如下：

properties：属性的名称

javaType：属性的类型

column：表中的外键字段名

select：使用另一个查询

二.实现一对多的联表查询

1.建立一个学生表，班级表与学生表示1：n的关系

sql脚本：

 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);

2.建立student实体类

package me.gacl.domain;

/**
 * 学生实体类
 */
public class Student {
    private int id;//id对应student表中的s_id
    private String name;//name对应student表中的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 + '\'' +
                '}';
    }
}

3.修改Classes类

类中添加学生对象

 package me.gacl.domain;

 import me.gacl.domain.Teacher;
 import java.util.List;
 /**
  * 班级实体类
  */
 public class Classes {
     private int id;//id对应c_id
     private String name;//name对应c_name
     private Teacher teacher;//因为class表中只有一个teacher_id外键，所以Class类中持有Teacher类的一个对象
     private List<Student> students;//因为班级与学生是1：n的关系，因此使用List存储一个班的n个学生

     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 +
                 '}';
     }
 }

4.编写映射文件

 <?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">
 <!--namespace一般命名成报名+映射名-->
 <mapper namespace="me.gacl.mapping.student_classMapper">
     <!--查询班级为1的班级对应的学生和教师信息-->
     <!--方法一：联表查询-->
    <select id="getClassTeacherStudent" parameterType="int" resultMap="classResultMap">
        select * from class,teacher,student where class.teacher_id=teacher.t_id and class.c_id=student.class_id and c_id=#{id}
    </select>
     <!--使用resultMap指定实体类字段与数据表属性之间的一一对应关系-->
     <resultMap id="classResultMap" type="Classes">
         <id column="c_id" property="id"></id>
         <result column="c_name" property="name"></result>
         <!--使用association中的javaType指定关联表中对应的实体类的类型，
         及实体类字段与数据表属性之间的一一对应关系-->
         <association property="teacher" column="teacher_id" javaType="Teacher">
             <id column="t_id" property="id"></id>
             <result column="t_name" property="name"></result>
         </association>
         <collection property="students" ofType="Student">
             <id column="s_id" property="id"></id>
             <result column="s_name" property="name"></result>
             <!--若没有指定属性class_id的对应column名称，则输出class_id为0
             也可以学生实体类中不包含属性class_id,那么打印输出中就没有class_id信息-->
             <result column="class_id" property="class_id"></result>
         </collection>
     </resultMap>
     <!--方法二，执行两步查询，引用另一步查询的查询结果-->
     <!--首先查询出班级为1对应的教师信息-->
     <select id="getClassTeacherStudent2" parameterType="int" resultMap="classResultMap2">
         select * from class where c_id=#{id}
     </select>
     <resultMap id="classResultMap2" type="Classes">
         <id column="c_id" property="id"></id>
         <result column="c_name" property="name"></result>
         <!--这里的column="teacher_id"一定不能少，因为teacher_id的值是作为参数传递给要引用的Select语句的
         相当于mybatis执行resultSet.getInt("teacher_id")，where后面的id入参就是得到的teacher_id的值-->
         <association property="teacher" column="teacher_id" select="getTeacher"></association>
         <!--这里的column="c_id"一定不能少，因为c_id的值是作为参数传递给要引用的Select语句的
         相当于mybatis执行resultSet.getInt("c_id")，where后面的id入参就是得到的c_id的值-->
         <collection property="students" column="c_id" select="getStudent"></collection>
     </resultMap>
     <!--然后根据上一步查询出的教师id到教师表中查询教师信息-->
     <select id="getTeacher" parameterType="int" resultType="Teacher">
         select t_id id,t_name name from teacher where t_id=#{id}
     </select>
     <!--然后根据第一步查询出的班级id到学生表中查询班级中的学生信息-->
     <select id="getStudent" parameterType="int" resultType="Student">
         select s_id id,s_name name from student where class_id=#{id}
     </select>
 </mapper>

5.mybatis_config.xml中注册student_classMapper映射文件

  <mappers>
         <mapper resource="mapping/student_classMapper.xml"></mapper>
     </mappers>

6.编写测试类

 package me.gacl.domain;

 import me.gacl.Util.MyBatisUtil;
 import org.apache.ibatis.session.SqlSession;

 /**
  * 测试一对多联表查询
  */
 public class Class_TeacherStudentTest {
     public void getClassTeacherStudent(){
         SqlSession sqlSession = MyBatisUtil.getSqlSession(true);
         String statement = "me.gacl.mapping.student_classMapper.getClassTeacherStudent";
         Classes classes = sqlSession.selectOne(statement,1);
         sqlSession.close();
         System.out.println(classes);
     }
     public void getClassTeacherStudent2(){
         SqlSession sqlSession = MyBatisUtil.getSqlSession(true);
         String statement = "me.gacl.mapping.student_classMapper.getClassTeacherStudent2";
         Classes classes = sqlSession.selectOne(statement,2);
         sqlSession.close();
         System.out.println(classes);
     }

     public static void main(String[] args) {
         Class_TeacherStudentTest class_teacherStudentTest = new Class_TeacherStudentTest();
         class_teacherStudentTest.getClassTeacherStudent();
         //class_teacherStudentTest.getClassTeacherStudent2();
     }
 }

测试结果：两个方法的输出结果一样

7.一对多关联查询总结

mybatis中解决一对多查询问题使用的是collection标签，属性ofType表示指定集合中元素的对象类型

注意： toString()是Java中的一个内置方法，如果你在类里编写了toString，相当于覆盖了类中原有的toString，在System.out.print中会自动调用，因此每个实体类的toString()方法都要正确，最好使用自动生成的，不然System.out.print打印结果不对