Mybatis之多對一處理

阿新 • • 發佈：2021-11-30

Mybatis

10、多對一處理

10.1、資料庫

USE ibatis;
CREATE TABLE teacher(
teacher_id INT(10) NOT NULL,
teacher_name VARCHAR(30) DEFAULT NULL,
PRIMARY KEY(teacher_id)
)ENGINE = INNOBASE DEFAULT CHARSET=utf8;
INSERT INTO teacher(teacher_id,teacher_name) VALUES (1,'jcooling');

CREATE TABLE student(
student_id INT(10) NOT NULL,
student_NAME VARCHAR(30) DEFAULT NULL,
teacher_id INT(10) DEFAULT NULL,
PRIMARY KEY(student_id),
KEY fkteacher_id (teacher_id),
CONSTRAINT fkteacher_id FOREIGN KEY(teacher_id) REFERENCES teacher(teacher_id)
)ENGINE = INNOBASE DEFAULT CHARSET=utf8;

INSERT INTO student(student_id,student_NAME,teacher_id) VALUES(1, '小明', '1');
INSERT INTO student(student_id,student_NAME,teacher_id) VALUES(2, '小紅', '1');
INSERT INTO student(student_id,student_NAME,teacher_id) VALUES(3, '小張', '1');
INSERT INTO student(student_id,student_NAME,teacher_id) VALUES(4, '小李', '1');
INSERT INTO student(student_id,student_NAME,teacher_id) VALUES(5, '小王', '1');

10.2、環境搭建

匯入lombok。

新建實體類Teacher，Student。

@NoArgsConstructor
@AllArgsConstructor
@Data
@ToString
public class Student {
    private int studentId;
    private String studentName;
    /*學生需要關聯一個老師！*/
    private Teacher teacher;
}

@NoArgsConstructor
@AllArgsConstructor
@Data
@ToString
public class Teacher {
    private Integer teacherId;
    private String teacherName;
}

建立Mapper介面。

```
public interface StudentMapper {
}
```

public interface TeacherMapper {
    Teacher getTeacher(@Param("teacherId") int teacherId);
}

寫*Mapper.xml。

<?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.jcooling.mapper.TeacherMapper">
     <select id="getTeacher" resultType="Teacher">
         select * from teacher where teacher_id= #{teacherId};
     </select>
</mapper>

建立*Mapper.xml檔案在核心配置檔案中繫結註冊我們的Mapper介面或者檔案！

<!--每一個Mapper.XML都需要在Mybatis核心配置檔案中註冊！-->
    <mappers>
       <package name="com.jcooling.mapper"></package>
    </mappers>

測試查詢是否能夠成功！

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

實體類屬性使用駝峰命名,為了能使資料庫欄位與Java實體類屬性一一對映,需要做一下配置。

<settings>
        <!--<setting name="logImpl" value="STDOUT_LOGGING"/>-->
        <setting name="mapUnderscoreToCamelCase" value="true"/>
 </settings>

10.3、按照查詢巢狀處理

  <!--思路：
    1.查詢所有的學生資訊。
    2.根據查詢出來的學生的teacher_id，尋找對應的老師！-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <!--<result property="studentId" column="student_id"></result>-->
        <!--<result property="studentName" column="student_name"></result>-->
        <!--複雜的屬性，我們需要單獨處理
        物件：association
        集合：colLection-->
        <association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacher"></association>
    </resultMap>
  
    <select id="getTeacher" resultType="Teacher">
        select * from teacher where teacher_id=#{teacherId}
    </select>

10.4、按照結果巢狀處理

 <!--按照結果巢狀處理-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.student_id sid,s.student_NAME sname,t.teacher_name tname from student s,teacher t where s.teacher_id=t.teacher_id;
    </select>
<resultMap id="StudentTeacher2" type="Student">
    <result property="studentId" column="sid" ></result>
    <result property="studentName" column="sname" ></result>
    <association property="teacher" javaType="Teacher">
        <result property="teacherName" column="tname"></result>
    </association>
</resultMap>