多对一和一对多的查询的ResultMap应用

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

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>

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>