MyBatis学习笔记五——实现关联表查询

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);

1.3、定义实体类

　　1、Teacher类，Teacher类是teacher表对应的实体类。
　　

package me.lizhenhua.domain;/** * @author lizhenhua * 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.lizhenhua.domain;/** * @author lizhenhua * 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.lizhenhua.mapping.classMapper"就是me.lizhenhua.mapping(包名)+classMapper(classMapper.xml文件去除后缀) --><mapper namespace="me.lizhenhua.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.lizhenhua.domain.Classes" id="ClassResultMap">        <id property="id" column="c_id"/>        <result property="name" column="c_name"/>        <association property="teacher" javaType="me.lizhenhua.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.lizhenhua.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.lizhenhua.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.lizhenhua.mapping这个包下，所以resource写成me/lizhenhua/mapping/classMapper.xml-->        <mapper resource="me/lizhenhua/mapping/classMapper.xml"/></mappers>

1.5、编写单元测试代码

package me.lizhenhua.test;import me.lizhenhua.domain.Classes;import me.lizhenhua.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.lizhenhua.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.lizhenhua.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.lizhenhua.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.lizhenhua.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:所二、一对多关联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.lizhenhua.domain;/** * @author lizhenhua * 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 students属性，使用一个List集合属性表示班级拥有的学生，如下：

package me.lizhenhua.domain;import java.util.List;/** * @author lizhenhua * 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.lizhenhua.domain.Classes" id="ClassResultMap3">        <id property="id" column="c_id"/>        <result property="name" column="c_name"/>        <association property="teacher" column="teacher_id" javaType="me.lizhenhua.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.lizhenhua.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.lizhenhua.domain.Classes" id="ClassResultMap4">        <id property="id" column="c_id"/>        <result property="name" column="c_name"/>        <association property="teacher" column="teacher_id" javaType="me.lizhenhua.domain.Teacher" select="getTeacher2"></association>        <collection property="students" ofType="me.lizhenhua.domain.Student" column="c_id" select="getStudent"></collection>     </resultMap>     <select id="getTeacher2" parameterType="int" resultType="me.lizhenhua.domain.Teacher">        SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}     </select>     <select id="getStudent" parameterType="int" resultType="me.lizhenhua.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.lizhenhua.test;import me.lizhenhua.domain.Classes;import me.lizhenhua.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.lizhenhua.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.lizhenhua.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.lizhenhua.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.lizhenhua.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。