MyBatis学习教程(五)-实现关联表查询方法详解

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()
 );
 CREATE TABLE class(
  c_id INT PRIMARY KEY AUTO_INCREMENT, 
  c_name VARCHAr(), 
  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('teacher');
 INSERT INTO teacher(t_name) VALUES('teacher');
 INSERT INTO class(c_name, teacher_id) VALUES('class_a', );
 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;
 
 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;
 
 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
  
  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

 
 
 
  
  
  
   select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
  
  
  
   
   
   
    
    
   
  
  
  
   select * from class where c_id=#{id}
  
  
  
   
   
   
  
  
   SELECT t_id id, t_name name FROM teacher WHERe t_id=#{id}
  
  

　　在conf.xml文件中注册classMapper.xml




  
  
 

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 Test {
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   
   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,);//查询class表中id为的记录
   //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=, name=class_a, teacher=Teacher [id=, name=teacher]]
  }
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   
   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,);//查询class表中id为的记录
   //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=, name=class_a, teacher=Teacher [id=, name=teacher]]
  }
 } 

 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;
 
 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.gacl.domain;
 import java.util.List;
 
 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
  
  private Teacher teacher;
  //使用一个List集合属性表示班级拥有的学生
  private List 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 getStudents() {
   return students;
  }
  public void setStudents(List 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




  
  
   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 * from class where c_id=#{id}
  
  
   
   
   
   
  
  
   SELECT t_id id, t_name name FROM teacher WHERe t_id=#{id}
  
  
   SELECT s_id id, s_name name FROM student WHERe class_id=#{id}
   

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 Test {
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   
   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,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   //打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, name=student_C]]]
   System.out.println(clazz);
  }
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   
   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,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   //打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, 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。

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