多对一和一对多的查询的ResultMap应用

1. 环境准备

   • 数据库创建：一个teacher表，字段有id(主键),name，一个student表，字段为id(主键)，name,tid(外键)
   • 建表语句：

   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, '王老师');
   
   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, '小明', '1'); 
   INSERT INTO `student` (`id`, `name`, `tid`) VALUES (2, '小红', '1'); 
   INSERT INTO `student` (`id`, `name`, `tid`) VALUES (3, '小张', '1'); 
   INSERT INTO `student` (`id`, `name`, `tid`) VALUES (4, '小李', '1'); 
   INSERT INTO `student` (`id`, `name`, `tid`) VALUES (5, '小王', '1');
   

   • 与表对应的实体类准备：

   //Teacher类
   public class Teacher {  
   	private Integer id;  
   	private String name;  
   	//...构造器、get、set方法等
   }
   //Student类
   public class Student {  
   	private Integer id;  
   	private String name;  
   	private Teacher teacher;  
   	//...构造器、get、set方法等
   }
   

   • 创建对应的mapper接口和mapper.xml文件，并在核心文件中配置mapper映射

2. 进行查询（学生相对于老师是多对一的关系）

   • 按照查询嵌套处理：

   //接口StudentMapper的处理
   public interface StudentMapper {
   
       //查询所有的学生信息，以及对应的老师的信息
       public List<Student> getStudent();
   }
   

   • StudentMapper.xml文件中写sql

   <?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">
   
   <mapper namespace="com.test.dao.StudentMapper">
   
       <!--
       思路：类似子查询
           1.查询所有的学生信息
           2.根据查询出来的学生的tid，寻找对应的老师
       -->
       
       <select id="getStudent" resultMap="StudentTeacher">
           select * from student;
       </select>
   
       <resultMap id="StudentTeacher" type="Student">
           <result property="id" column="id"/>
           <result property="name" column="name"/>
           <!--复杂的属性，需要单独处理，对象：association，集合：collection-->
           <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
       </resultMap>
   
       <select id="getTeacher" resultType="Teacher">
           select * from teacher where id = #{id};
       </select>
   </mapper>
   

   • 按照结果嵌套处理:连表查询的思路

   <?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">
   
   <mapper namespace="com.test.dao.StudentMapper">
   
       <!--按照结果嵌套处理-->
       <select id="getStudent" resultMap="StudentTeacher">
           select s.id sid,s.name sname,t.id tid,t.name tname
           from student s,teacher t
           where  s.tid = t.id;
       </select>
   
       <resultMap id="StudentTeacher" type="Student">
           <result property="id" column="sid"/>
           <result property="name" column="sname"/>
           <association property="teacher" javaType="Teacher">
               <result property="name" column="tname"/>
               <result property="id" column="tid"/>
           </association>
       </resultMap>
   </mapper>
   

3. 老师相对于学生是一对多的关系

   • 实体类的编写

   @Data
   public class Student {
       private int id;
       private String name;
       private int tid;
   }
   @Data
   public class Teacher {
       private int id;
       private String name;
       private List<Student> students;
   }
   

   • 编写mapper.xml中的sql语句

   <?xml version="1.0" encoding="UTF-8" ?>
   <!DOCTYPE mapper
           PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
   <mapper namespace="com.test.dao.TeacherMapper">
    
       <!--按结果嵌套查询-->
       <select id="getTeacher" resultMap="TeacherStudent">
           select s.id sid, s.name sname, t.id tid, t.name tname
           from student s,
                teacher t
           where s.tid = t.id
             and t.id = #{tid}
       </select>
       <resultMap id="TeacherStudent" type="Teacher">
           <result property="id" column="tid"/>
           <result property="name" column="tname"/>
           <!--ofType="" 用于获取集合中泛型的信息-->
           <collection property="students" ofType="Student">
               <result property="id" column="sid"/>
               <result property="name" column="sname"/>
               <result property="tid" column="tid"/>
           </collection>
       </resultMap>
   
       <!--嵌套表查询（子查询）-->
       <select id="getTeacher1" resultMap="TeacherStudent1">
           select * from teacher where id=#{tid}
       </select>
       <resultMap id="TeacherStudent1" type="Teacher">
           <result property="id" column="id"/>
           <collection property="student" javaType="ArrayList" ofType="Student" select="getStudetByID" column="id"/>
       </resultMap>
       <select id="getStudetByID" resultType="Student">
           select * from student where tid=#{tid}
       </select>
   </mapper>