MyBatis ORM映射和多表连接查询

待我称王封你为后i 2023-02-24 08:42 47阅读 0赞

### MyBatis ORM映射和多表连接查询 ###

*   *   *  1. ORM映射【★】
         *   *  1.1 MyBatis自动ORM失效
             *  1.2 一：列的别名
             *  1.3 二：结果映射
         *  2. MyBatis处理关联关系-多表连接【★】
         *   *  2.1 OneToOne
             *  2.2 OneToMany
             *  2.3 ManyToMany
             *  2.4 关系总结

![001][]

### 1. ORM映射【★】 ###

#### 1.1 MyBatis自动ORM失效 ####

MyBatis 只能自动维护库表`列名与属性名相同`时的一一对应关系，二者不同时，无法自动ORM。

<table> 
 <thead> 
  <tr> 
   <th align="center">自动ORM失效</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9qeS1pbWdzLm9zcy1jbi1iZWlqaW5nLmFsaXl1bmNzLmNvbS9pbWcvMDA4LnBuZw?x-oss-process=image/format,png" alt="007"></td> 
  </tr> 
 </tbody> 
</table>

#### 1.2 一：列的别名 ####

在SQL中使用 `as` 为查询字段添加列别名，以匹配属性名。

> 注意：列名与属性名相同时可不需要添加 列别名，直接使用 **ResultMap** 结果映射。

<mapper namespace="com.mybatis.part2.orm.ManagerDao">
        <select id="selectManagerByIdAndPwd" resultType="com.mybatis.part2.orm.Manager">
            SELECT mgr_id AS id , mgr_name AS username , mgr_pwd AS password
            FROM t_managers
            WHERE mgr_id = #{id} AND mgr_pwd = #{pwd}
        </select>
    </mapper>

#### 1.3 二：结果映射 ####

**ResultMap** \- 查询结果的封装规则，通过`<resultMap id="" type="">`映射，匹配列名与属性名。

> 注意：
> 
> 可通过 `<settings>` 标签将 ORM 映射结果的自动映射行为配置为 `FULL` ，此时只会映射 列名与属性名 相同的值，`对象类型 和 集合类型 还需手动映射`。

mybatis-config.xml

Mapper.xml

<mapper namespace="com.mybatis.part2.orm.ManagerDao">
    
        
        <resultMap id="managerResultMap" type="com.mybatis.part2.orm.Manager">
          	
            <id property="id" column="mgr_id" />
    
          	
            <result property="username" column="mgr_name" />
            <result property="password" column="mgr_pwd" />
        </resultMap>
      
         
        <select id="selectAllManagers" resultMap="managerResultMap">
            SELECT mgr_id , mgr_name , mgr_pwd
            FROM t_managers
        </select>
    </mapper>

### 2. MyBatis处理关联关系-多表连接【★】 ###

实体间的关系：关联关系（拥有 has、属于 belong）

*  **OneToOne**：一对一关系（Passenger旅客 — Passport护照）
 *  **OneToMany**：一对多关系（Employee员工 — Department部门）
 *  **ManyToMany**：多对多关系（Student学生 — Subject科目）

<table> 
 <thead> 
  <tr> 
   <th align="center">Table建立外键关系</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9qeS1pbWdzLm9zcy1jbi1iZWlqaW5nLmFsaXl1bmNzLmNvbS9pbWcvaW1hZ2UtMjAxOTEyMzAxNzQ3Mjc4MTMucG5n?x-oss-process=image/format,png" alt="008"></td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <thead> 
  <tr> 
   <th align="center">Entity添加关系属性</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9qeS1pbWdzLm9zcy1jbi1iZWlqaW5nLmFsaXl1bmNzLmNvbS9pbWcvMDA5XzIuanBn?x-oss-process=image/format,png" alt="009_2"></td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <thead> 
  <tr> 
   <th align="center">Mapper中将属性与列名对应</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9qeS1pbWdzLm9zcy1jbi1iZWlqaW5nLmFsaXl1bmNzLmNvbS9pbWcvMDEwLnBuZw?x-oss-process=image/format,png" alt="010"></td> 
  </tr> 
 </tbody> 
</table>

#### 2.1 OneToOne ####

<mapper namespace="com.mybatis.part2.one2one.PassengerDao">
    
      	
        <resultMap id="passengerResultMap" type="com.mybatis.part2.one2one.Passenger">
            <id property="id" column="id"/>
            <result property="name" column="name" />
            <result property="sex" column="sex" />
            <result property="birthday" column="birthday" />
    
          		 
            <association property="passport" javaType="com.mybatis.part2.one2one.Passport">
                <id property="id" column="passport_id" />
                <result property="nationality" column="nationality" />
                <result property="expire" column="expire" />
              	<result property="passenger_id" column="passenger_id" />
            </association>
        </resultMap>
    
      		 
        <select id="selectPassengerById" resultMap="passengerResultMap">
            
            SELECT p1.id , p1.name , p1.sex , p1.birthday , p2.id as passport_id , p2.nationality , p2.expire , p2.passenger_id
            FROM t_passengers p1 LEFT JOIN t_passports p2
            ON p1.id = p2.passenger_id
            WHERE p1.id = #{id}
        </select>
    </mapper>

*  注意：指定“一方”关系时（对象），使用 `<association javaType="">`

#### 2.2 OneToMany ####

<mapper namespace="com.mybatis.part2.one2many.DepartmentDao">
    
      	
        <resultMap id="departmentResultMap" type="com.mybatis.part2.one2many.Department">
            <id property="id" column="id" />
            <result property="name" column="name" />
            <result property="location" column="location" />
            
          			
            <collection property="emps" ofType="com.mybatis.part2.one2many.Employee">
                <id property="id" column="emp_id" />
                <result property="name" column="emp_name" />
                <result property="salary" column="salary" />
                <result property="dept_id" column="dept_id" />
            </collection>
        </resultMap>
    
      				      
        <select id="selectDepartmentById" resultMap="departmentResultMap" >
          	
            SELECT d.id , d.name , d.location , e.id AS emp_id , e.name emp_name , e.salary , e.dept_id
            FROM t_departments d LEFT JOIN t_employees e
            ON d.id = e.dept_id
            WHERE d.id = #{id}
        </select>
    
    </mapper>

*  注意：指定“多方”关系时（集合），使用 `<collection ofType="">`

#### 2.3 ManyToMany ####

<table> 
 <thead> 
  <tr> 
   <th align="center">建立第三张关系表</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="center"><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9qeS1pbWdzLm9zcy1jbi1iZWlqaW5nLmFsaXl1bmNzLmNvbS9pbWcvMDExXzIuanBn?x-oss-process=image/format,png" alt="011_2"></td> 
  </tr> 
 </tbody> 
</table>

<mapper namespace="com.mybatis.part2.many2many.StudentDao">
    
      	
        <resultMap id="allMap" type="com.mybatis.part2.many2many.Student">
            <id property="id" column="id" />
            <result property="name" column="name" />
            <result property="sex" column="sex" />
            <collection property="subjects" ofType="com.mybatis.part2.many2many.Subject">
                <id property="id" column="sid" />
                <result property="name" column="sname" />
                <result property="grade" column="grade" />
            </collection>
        </resultMap>
    
      	
        <select id="selectAllStudents" resultMap="allMap">
            SELECT s1.* , ss.* , s2.id as sid , s2.name as sname , s2.grade
            FROM t_students s1 LEFT JOIN t_stu_sub ss
            ON s1.id = ss.student_id 
            LEFT JOIN t_subjects s2
            ON ss.subject_id = s2.id
        </select>
    </mapper>

*  注意：指定“多方”关系时（集合），使用 `<collection ofType="">`

#### 2.4 关系总结 ####

一方，属性字段中添加集合；多方，属性字段中添加对象。

双方均可建立关系属性，建立关系属性后，对应的 Mapper 文件中需使用 `<ResultMap>` 完成多表映射。

*  持有对象关系属性，使用 `<association property="dept" javaType="department">`
 *  持有集合关系属性，使用 `<collection property="emps" ofType="employee">`

[001]: /images/20230209/16c54bcf0533483daa5b12eb8c0225bd.png