MyBatis学习总结(五)——实现关联表查询

谁践踏了优雅 2022-10-02 08:47 177阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

## 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。

![复制代码][31085614_rVcG.gif]

1 CREATE TABLE teacher(
     2     t_id INT PRIMARY KEY AUTO_INCREMENT, 
     3     t_name VARCHAR(20)
     4 );
     5 CREATE TABLE class(
     6     c_id INT PRIMARY KEY AUTO_INCREMENT, 
     7     c_name VARCHAR(20), 
     8     teacher_id INT
     9 );
    10 ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);    
    11 
    12 INSERT INTO teacher(t_name) VALUES('teacher1');
    13 INSERT INTO teacher(t_name) VALUES('teacher2');
    14 
    15 INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
    16 INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);

![复制代码][31085614_rVcG.gif]

表之间的关系如下：

![31085614_mwgM.png][]

### 1.3、定义实体类 ###

1、Teacher类，Teacher类是teacher表对应的实体类。

![复制代码][31085614_rVcG.gif]

1 package me.gacl.domain;
     2 
     3 /**
     4  * @author gacl
     5  * 定义teacher表对应的实体类
     6  */
     7 public class Teacher {
     8 
     9     //定义实体类的属性，与teacher表中的字段对应
    10     private int id;            //id===>t_id
    11     private String name;    //name===>t_name
    12 
    13     public int getId() {
    14         return id;
    15     }
    16 
    17     public void setId(int id) {
    18         this.id = id;
    19     }
    20 
    21     public String getName() {
    22         return name;
    23     }
    24 
    25     public void setName(String name) {
    26         this.name = name;
    27     }
    28 
    29     @Override
    30     public String toString() {
    31         return "Teacher [id=" + id + ", name=" + name + "]";
    32     }
    33 }

![复制代码][31085614_rVcG.gif]

2、Classes类，Classes类是class表对应的实体类

![复制代码][31085614_rVcG.gif]

1 package me.gacl.domain;
     2 
     3 /**
     4  * @author gacl
     5  * 定义class表对应的实体类
     6  */
     7 public class Classes {
     8 
     9     //定义实体类的属性，与class表中的字段对应
    10     private int id;            //id===>c_id
    11     private String name;    //name===>c_name
    12     
    13     /**
    14      * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
    15      * 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
    16      */
    17     private Teacher teacher;
    18 
    19     public int getId() {
    20         return id;
    21     }
    22 
    23     public void setId(int id) {
    24         this.id = id;
    25     }
    26 
    27     public String getName() {
    28         return name;
    29     }
    30 
    31     public void setName(String name) {
    32         this.name = name;
    33     }
    34 
    35     public Teacher getTeacher() {
    36         return teacher;
    37     }
    38 
    39     public void setTeacher(Teacher teacher) {
    40         this.teacher = teacher;
    41     }
    42 
    43     @Override
    44     public String toString() {
    45         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
    46     }
    47 }

![复制代码][31085614_rVcG.gif]

### 1.4、定义sql映射文件classMapper.xml ###

![复制代码][31085614_rVcG.gif]

1 <?xml version="1.0" encoding="UTF-8" ?>
     2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
     3 
     6 <mapper namespace="me.gacl.mapping.classMapper">
     7 
     8     
    17 
    18     
    23     <select id="getClass" parameterType="int" resultMap="ClassResultMap">
    24         select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
    25     </select>
    26     
    27     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
    28         <id property="id" column="c_id"/>
    29         <result property="name" column="c_name"/>
    30         <association property="teacher" javaType="me.gacl.domain.Teacher">
    31             <id property="id" column="t_id"/>
    32             <result property="name" column="t_name"/>
    33         </association>
    34     </resultMap>
    35     
    36     
    41      <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
    42         select * from class where c_id=#{id}
    43      </select>
    44      
    45      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
    46         <id property="id" column="c_id"/>
    47         <result property="name" column="c_name"/>
    48         <association property="teacher" column="teacher_id" select="getTeacher"/>
    49      </resultMap>
    50      
    51      <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
    52         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
    53      </select>
    54 
    55 </mapper>

![复制代码][31085614_rVcG.gif]

在conf.xml文件中注册classMapper.xml

### 1.5、编写单元测试代码 ###

![复制代码][31085614_rVcG.gif]

1 package me.gacl.test;
     2 
     3 import me.gacl.domain.Classes;
     4 import me.gacl.util.MyBatisUtil;
     5 import org.apache.ibatis.session.SqlSession;
     6 import org.junit.Test;
     7 
     8 public class Test3 {
     9     
    10     @Test
    11     public void testGetClass(){
    12         SqlSession sqlSession = MyBatisUtil.getSqlSession();
    13         /**
    14          * 映射sql的标识字符串，
    15          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
    16          * 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
    17          */
    18         String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
    19         //执行查询操作，将查询结果自动封装成Classes对象返回
    20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
    21         //使用SqlSession执行完SQL之后需要关闭SqlSession
    22         sqlSession.close();
    23         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
    24     }
    25     
    26     @Test
    27     public void testGetClass2(){
    28         SqlSession sqlSession = MyBatisUtil.getSqlSession();
    29         /**
    30          * 映射sql的标识字符串，
    31          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
    32          * 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
    33          */
    34         String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的标识字符串
    35         //执行查询操作，将查询结果自动封装成Classes对象返回
    36         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
    37         //使用SqlSession执行完SQL之后需要关闭SqlSession
    38         sqlSession.close();
    39         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
    40     }
    41 }

![复制代码][31085614_rVcG.gif]

### 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

![复制代码][31085614_rVcG.gif]

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);

![复制代码][31085614_rVcG.gif]

![31085614_KyND.png][]

### 2.3、定义实体类 ###

1、Student类

![复制代码][31085614_rVcG.gif]

1 package me.gacl.domain;
     2 
     3 /**
     4  * @author gacl
     5  * 定义student表所对应的实体类
     6  */
     7 public class Student {
     8 
     9     //定义属性，和student表中的字段对应
    10     private int id;            //id===>s_id
    11     private String name;    //name===>s_name
    12     
    13     public int getId() {
    14         return id;
    15     }
    16 
    17     public void setId(int id) {
    18         this.id = id;
    19     }
    20 
    21     public String getName() {
    22         return name;
    23     }
    24 
    25     public void setName(String name) {
    26         this.name = name;
    27     }
    28 
    29     @Override
    30     public String toString() {
    31         return "Student [id=" + id + ", name=" + name + "]";
    32     }
    33 }

![复制代码][31085614_rVcG.gif]

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：

![复制代码][31085614_rVcG.gif]

1 package me.gacl.domain;
     2 
     3 import java.util.List;
     4 
     5 /**
     6  * @author gacl
     7  * 定义class表对应的实体类
     8  */
     9 public class Classes {
    10 
    11     //定义实体类的属性，与class表中的字段对应
    12     private int id;            //id===>c_id
    13     private String name;    //name===>c_name
    14     
    15     /**
    16      * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
    17      * 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
    18      */
    19     private Teacher teacher;
    20     //使用一个List<Student>集合属性表示班级拥有的学生
    21     private List<Student> students;
    22 
    23     public int getId() {
    24         return id;
    25     }
    26 
    27     public void setId(int id) {
    28         this.id = id;
    29     }
    30 
    31     public String getName() {
    32         return name;
    33     }
    34 
    35     public void setName(String name) {
    36         this.name = name;
    37     }
    38 
    39     public Teacher getTeacher() {
    40         return teacher;
    41     }
    42 
    43     public void setTeacher(Teacher teacher) {
    44         this.teacher = teacher;
    45     }
    46 
    47     public List<Student> getStudents() {
    48         return students;
    49     }
    50 
    51     public void setStudents(List<Student> students) {
    52         this.students = students;
    53     }
    54 
    55     @Override
    56     public String toString() {
    57         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
    58                 + ", students=" + students + "]";
    59     }
    60 }

![复制代码][31085614_rVcG.gif]

### 2.4、修改sql映射文件classMapper.xml ###

添加如下的SQL映射信息

![复制代码][31085614_rVcG.gif]

1 
     4     
     8     <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
     9         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}
    10     </select>
    11     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
    12         <id property="id" column="c_id"/>
    13         <result property="name" column="c_name"/>
    14         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
    15             <id property="id" column="t_id"/>
    16             <result property="name" column="t_name"/>
    17         </association>
    18         
    19         <collection property="students" ofType="me.gacl.domain.Student">
    20             <id property="id" column="s_id"/>
    21             <result property="name" column="s_name"/>
    22         </collection>
    23     </resultMap>
    24     
    25     
    31      <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
    32         select * from class where c_id=#{id}
    33      </select>
    34      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
    35         <id property="id" column="c_id"/>
    36         <result property="name" column="c_name"/>
    37         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
    38         <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
    39      </resultMap>
    40      
    41      <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
    42         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
    43      </select>
    44      
    45      <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
    46         SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
    47      </select>

![复制代码][31085614_rVcG.gif]

### 2.5、编写单元测试代码 ###

![复制代码][31085614_rVcG.gif]

1 package me.gacl.test;
     2 
     3 import me.gacl.domain.Classes;
     4 import me.gacl.util.MyBatisUtil;
     5 import org.apache.ibatis.session.SqlSession;
     6 import org.junit.Test;
     7 
     8 public class Test4 {
     9     
    10     @Test
    11     public void testGetClass3(){
    12         SqlSession sqlSession = MyBatisUtil.getSqlSession();
    13         /**
    14          * 映射sql的标识字符串，
    15          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
    16          * 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
    17          */
    18         String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串
    19         //执行查询操作，将查询结果自动封装成Classes对象返回
    20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
    21         //使用SqlSession执行完SQL之后需要关闭SqlSession
    22         sqlSession.close();
    23         //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]]]
    24         System.out.println(clazz);
    25     }
    26     
    27     @Test
    28     public void testGetClass4(){
    29         SqlSession sqlSession = MyBatisUtil.getSqlSession();
    30         /**
    31          * 映射sql的标识字符串，
    32          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
    33          * 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
    34          */
    35         String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串
    36         //执行查询操作，将查询结果自动封装成Classes对象返回
    37         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
    38         //使用SqlSession执行完SQL之后需要关闭SqlSession
    39         sqlSession.close();
    40         //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]]]
    41         System.out.println(clazz);
    42     }
    43 }

![复制代码][31085614_rVcG.gif]

### 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。

转载于:https://my.oschina.net/zhanghaiyang/blog/593725

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220113/4378fc0d341142b0a04b561dd1e1bc00.png
[31085614_rVcG.gif]: /images/20220113/fee62892ce0a42018aae89ecf9d8992a.png
[31085614_mwgM.png]: /images/20220113/7e6cc4edf0f74bdc822343229f4e8a08.png
[31085614_KyND.png]: /images/20220113/f638a47db74d4b8e8f95ee35d2389ff6.png