MyBatis学习总结(五)——实现关联表查询

jopen 6年前

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);

表之间的关系如下:


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]]      }  }

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);



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。





来自: http://blog.csdn.net/hejingyuan6/article/details/48689571