MyBatis复杂sql：多对一处理（association）和一对多（collection）

多对一（association标签）

• 老师和学生的例子
• 以学生为出发点，就是一个多对一的例子，即多个学生关联一个老师！

首先搭建数据库

CREATE TABLE `teacher` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO teacher(`id`, `name`) VALUES (1, '羊羊');

CREATE TABLE `student` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8


INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小羊', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小锅', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小虎', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小狗', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小明', '1');

利用lombok简化代码

1、Lombok

• lombok是一个可以帮助我们简化java代码编写的工具类，尤其是简化javabean的编写，即通过采用注解的方式，消除代码中的构造方法，getter/setter等代码，使我们写的类更加简洁，当然，这带来的副作用就是不易阅读…
• lombok注解解释：https://www.cnblogs.com/heyonggang/p/8638374.html，当然大家也可以自行百度

2、引入Lombok的Maven依赖

<dependency>
 <groupId>org.projectlombokgroupId>
 <artifactId>lombokartifactId>
 <version>1.16.10version>
dependency>

3、在代码中增加注解

@Data //GET,SET,ToString，有参，无参构造
public class Teacher {
   private int id;
   private String name;
}
@Data
public class Student {
   private int id;
   private String name;
   //多个学生可以是同一个老师，即多对一
   private Teacher teacher;
}

4、编写实体类对应的Mapper接口

public interface StudentMapper {
}
public interface TeacherMapper {
}

5、编写Mapper接口对应的 mapper.xml配置文件

<mapper namespace="com.yang.mapper.StudentMapper">

mapper>


<mapper namespace="com.yang.mapper.TeacherMapper">

mapper>

复杂查询，获取所有学生及对应老师的信息

• 两种方法实现
• 按查询嵌套处理（像SQL中的子查询）
• 按结果嵌套处理（像SQL中的联表查询）

1、给StudentMapper接口增加方法

public List<Student> getStudents();   //查询嵌套处理
public List<Student> getStudents2();  //结果嵌套处理

2、编写对应的Mapper文件

<mapper namespace="com.yang.mapper.StudentMapper">

   
   <select id="getStudents" resultMap="StudentTeacher">
    select * from student
   select>
   <resultMap id="StudentTeacher" type="Student">
       
       
   resultMap>
   
   <select id="getTeacher" resultType="teacher">
      select * from teacher where id = #{id}
   select>
   
	
	<select id="getStudents2" resultMap="StudentTeacher2" >
	  select s.id sid, s.name sname , t.name tname
	  from student s,teacher t
	  where s.tid = t.id
	select>
	
	<resultMap id="StudentTeacher2" type="Student">
	   <id property="id" column="sid"/>
	   <result property="name" column="sname"/>
	   
	   <association property="teacher" javaType="Teacher">
	       <result property="name" column="tname"/>
	   association>
	resultMap>

mapper>

3、记得注册Mapper

4、注意点说明：

<resultMap id="StudentTeacher" type="Student">
   
   
resultMap>

<select id="getTeacher" resultType="teacher">
  select * from teacher where id = #{id} and name = #{name}
select>

5、测试

@Test
public void testGetStudents(){
   SqlSession session = MybatisUtils.getSession();
   StudentMapper mapper =session.getMapper(StudentMapper.class);

   List<Student> students = mapper.getStudents();

   for (Student student : students){
       System.out.println(
               "学生名:"+ student.getName()
                       +"\t老师:"+student.getTeacher().getName());
  }
}

@Test
public void testGetStudents2(){
   SqlSession session = MybatisUtils.getSession();
   StudentMapper mapper =session.getMapper(StudentMapper.class);

   List<Student> students = mapper.getStudents2();

   for (Student student : students){
       System.out.println(
               "学生名:"+ student.getName()
                       +"\t老师:"+student.getTeacher().getName());
  }
}

一对多的处理

• 老师和学生的例子
• 以老师为出发点，就是一个多对一的例子，即一个老师关联多个学生！

环境相同，使用两种方法分别实现
按结果嵌套处理

1、TeacherMapper接口编写方法

//获取指定老师，及老师下的所有学生
public Teacher getTeacher(int id);

2、编写接口对应的Mapper配置文件

<mapper namespace="com.yang.mapper.TeacherMapper">

   
   <select id="getTeacher" resultMap="TeacherStudent">
      select s.id sid, s.name sname , t.name tname, t.id tid
      from student s,teacher t
      where s.tid = t.id and t.id=#{id}
   select>

   <resultMap id="TeacherStudent" type="Teacher">
       <result  property="name" column="tname"/>
       <collection property="students" ofType="Student">
           <result property="id" column="sid" />
           <result property="name" column="sname" />
           <result property="tid" column="tid" />
       collection>
   resultMap>
mapper>

3、将Mapper文件注册到MyBatis-config文件中

<mappers>
   <mapper resource="mapper/TeacherMapper.xml"/>
mappers>

4、测试

@Test
public void testGetTeacher(){
   SqlSession session = MybatisUtils.getSession();
   TeacherMapper mapper =session.getMapper(TeacherMapper.class);
   Teacher teacher = mapper.getTeacher(1);
   System.out.println(teacher.getName());
   System.out.println(teacher.getStudents());
}

按查询嵌套处理

1、TeacherMapper接口编写方法

public Teacher getTeacher2(int id);

2、编写接口对应的Mapper配置文件

<select id="getTeacher2" resultMap="TeacherStudent2">
select * from teacher where id = #{id}
select>
<resultMap id="TeacherStudent2" type="Teacher">
   
   
resultMap>
<select id="getStudentByTeacherId" resultType="Student">
  select * from student where tid = #{id}
select>

3、将Mapper文件注册到MyBatis-config文件中

4、测试

@Test
public void testGetTeacher2(){
   SqlSession session = MybatisUtils.getSession();
   TeacherMapper mapper =session.getMapper(TeacherMapper.class);
   Teacher teacher = mapper.getTeacher2(1);
   System.out.println(teacher.getName());
   System.out.println(teacher.getStudents());
}