SQL子查询与MyBatis映射过程

<!-- 结果映射 -->
<mapper namespace="com.example.dao.UserMapper">
	<resultMap id="userResultMap" type="实体类的全限定名">
	  <result property="userId" column="USER_ID"/>
	  <result property="username" column="USER_NAME"/>
	  <result property="departmentName" column="DEPARTMENT_NAME"/>
	  <result property="orderCount" column="ORDER_COUNT"/>
	  <result property="createTime" column="CREATE_TIME"/>
	</resultMap>
</mapper>
<!-- 定义可复用的基础字段 -->
<sql id="baseUserColumns">
  u.id AS USER_ID,
  u.username AS USER_NAME,
  u.create_time AS CREATE_TIME
</sql>
<!-- 定义部门关联字段 -->
<sql id="deptColumns">
  d.dept_name AS DEPARTMENT_NAME
</sql>
<!-- 复合查询 -->
<select id="selectUserWithStats" resultMap="userResultMap">
  SELECT 
    <!-- 复用基础字段 -->
    <include refid="baseUserColumns"/>,
    <!-- 复用部门字段 -->
    <include refid="deptColumns"/>,
    <!-- 使用子查询和别名 -->
    (
      SELECT COUNT(*) 
      FROM `order` o 
      WHERE o.user_id = u.id
      AND o.order_time >= DATE_SUB(NOW(), INTERVAL 3 MONTH)
    ) AS ORDER_COUNT
  FROM user u
  LEFT JOIN department d ON u.dept_id = d.id
  WHERE u.id = #{userId}
</select>

public class UserDTO {
    private Long userId;               // 映射 USER_ID
    private String username;           // 映射 USER_NAME
    private String departmentName;     // 映射 DEPARTMENT_NAME
    private Integer orderCount;        // 映射 ORDER_COUNT
    private Date createTime;           // 映射 CREATE_TIME

    // Getters and Setters
}

(SELECT COUNT(*) ...) AS ORDER_COUNT

<resultMap id="userResultMap" type="UserDTO">
  <id property="userId" column="USER_ID"/>
  <result property="departmentName" column="DEPARTMENT_NAME"/>
</resultMap>

<include refid="baseUserColumns"/>
<include refid="deptColumns"/> 

SELECT 
  U.ID AS USER_ID,
  U.USERNAME AS USER_NAME,
  U.CREATE_TIME AS CREATE_TIME,
  D.DEPT_NAME AS DEPARTMENT_NAME,
  (SELECT COUNT(*) 
   FROM `order` O 
   WHERE O.USER_ID = U.ID
   AND O.ORDER_TIME >= DATE_SUB(CURRENT_DATE(), INTERVAL 3 MONTH)) AS ORDER_COUNT
FROM USER U
LEFT JOIN DEPARTMENT D ON U.DEPT_ID = D.ID
WHERE U.ID = #{userId}

(SELECT COUNT(*) 
 FROM `order` o 
 WHERE o.user_id = u.id
 AND o.order_time >= DATE_SUB(CURRENT_DATE(), INTERVAL 3 MONTH)) AS ORDER_COUNT

UserDTO{
  userId=1001, 
  username="张三", 
  departmentName="技术部", 
  orderCount=5, 
  createTime=2023-01-15 // 假设时间为2023年1月15日
}