SQL 语句题 | 重新格式化部门表

题目

部门表 Department：

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| revenue       | int     |
| month         | varchar |
+---------------+---------+

(id, month) 是表的联合主键。
这个表格有关于每个部门每月收入的信息。
月份（month）可以取下列值

["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]。

编写一个 SQL 查询来重新格式化表，使得新的表中有一个部门 id 列和一些对应 每个月 的收入（revenue）列。

查询结果格式如下面的示例所示：

Department 表：
+------+---------+-------+
| id   | revenue | month |
+------+---------+-------+
| 1    | 8000    | Jan   |
| 2    | 9000    | Jan   |
| 3    | 10000   | Feb   |
| 1    | 7000    | Feb   |
| 1    | 6000    | Mar   |
+------+---------+-------+

查询得到的结果表：

+------+-------------+-------------+-------------+-----+-------------+
| id   | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1    | 8000        | 7000        | 6000        | ... | null        |
| 2    | 9000        | null        | null        | ... | null        |
| 3    | null        | 10000       | null        | ... | null        |
+------+-------------+-------------+-------------+-----+-------------+

注意，结果表有 13 列 (1个部门 id 列 + 12个月份的收入列)

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reformat-department-table

解析

第一步 列拆成行

首先，根据题意，需要将revenue列根据month拆成行，结果如下表，有两种方式，CASE WHEN或者IF语句。

+------+-------------+-------------+-------------+-----+-------------+
| id   | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1    | null        | 7000        | null       | ... | null        |

| 3    | null        | 10000       | null        | ... | null        |
| 1    | 8000        | null        | null        | ... | null        |
| 2    | 9000        | null        | null        | ... | null        |
| 1    | null        | null        | 6000        | ... | null        |
+------+-------------+-------------+-------------+-----+-------------+

CASE WHEN

SELECT id,
CASE `month` WHEN 'Jan' THEN revenue END Jan_Revenue,
CASE `month` WHEN 'Feb' THEN revenue END Feb_Revenue,
CASE `month` WHEN 'Mar' THEN revenue END Mar_Revenue,
CASE `month` WHEN 'Apr' THEN revenue END Apr_Revenue,
CASE `month` WHEN 'May' THEN revenue END May_Revenue,
CASE `month` WHEN 'Jun' THEN revenue END Jun_Revenue,
CASE `month` WHEN 'Jul' THEN revenue END Jul_Revenue,
CASE `month` WHEN 'Aug' THEN revenue END Aug_Revenue,
CASE `month` WHEN 'Sep' THEN revenue END Sep_Revenue,
CASE `month` WHEN 'Oct' THEN revenue END Oct_Revenue,
CASE `month` WHEN 'Nov' THEN revenue END Nov_Revenue,
CASE `month` WHEN 'Dec' THEN revenue END Dec_Revenue
FROM Department;

IF

SELECT id,
IF(`month`='Jan',revenue,NULL) Jan_Revenue,
IF(`month`='Feb',revenue,NULL) Feb_Revenue,
IF(`month`='Mar',revenue,NULL) Mar_Revenue,
IF(`month`='Apr',revenue,NULL) Apr_Revenue,
IF(`month`='May',revenue,NULL) May_Revenue,
IF(`month`='Jun',revenue,NULL) Jun_Revenue,
IF(`month`='Jul',revenue,NULL) Jul_Revenue,
IF(`month`='Aug',revenue,NULL) Aug_Revenue,
IF(`month`='Sep',revenue,NULL) Sep_Revenue,
IF(`month`='Oct',revenue,NULL) Oct_Revenue,
IF(`month`='Nov',revenue,NULL) Nov_Revenue,
IF(`month`='Dec',revenue,NULL) Dec_Revenue
FROM Department;

第二步 行合并

其次，根据题意，需要根据id，将相同id的行进行合并，结果如下表，采用分组的方式，可以对各月收入列求和（SUM）也可以求最大值(MAX)。

+------+-------------+-------------+-------------+-----+-------------+
| id   | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1    | 8000        | 7000        | 6000        | ... | null        |
| 2    | 9000        | null        | null        | ... | null        |
| 3    | null        | 10000       | null        | ... | null        |
+------+-------------+-------------+-------------+-----+-------------+

解法

SELECT id,
SUM(CASE `month` WHEN 'Jan' THEN revenue END) Jan_Revenue,
SUM(CASE `month` WHEN 'Feb' THEN revenue END) Feb_Revenue,
SUM(CASE `month` WHEN 'Mar' THEN revenue END) Mar_Revenue,
SUM(CASE `month` WHEN 'Apr' THEN revenue END) Apr_Revenue,
SUM(CASE `month` WHEN 'May' THEN revenue END) May_Revenue,
SUM(CASE `month` WHEN 'Jun' THEN revenue END) Jun_Revenue,
SUM(CASE `month` WHEN 'Jul' THEN revenue END) Jul_Revenue,
SUM(CASE `month` WHEN 'Aug' THEN revenue END) Aug_Revenue,
SUM(CASE `month` WHEN 'Sep' THEN revenue END) Sep_Revenue,
SUM(CASE `month` WHEN 'Oct' THEN revenue END) Oct_Revenue,
SUM(CASE `month` WHEN 'Nov' THEN revenue END) Nov_Revenue,
SUM(CASE `month` WHEN 'Dec' THEN revenue END) Dec_Revenue
FROM Department
GROUP BY id;