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。
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”]。
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)。
解答
1SELECT
2 id,
3 SUM(IF(month="Jan", revenue, null)) as Jan_Revenue,
4 SUM(IF(month="Feb", revenue, null)) as Feb_Revenue,
5 SUM(IF(month="Mar", revenue, null)) as Mar_Revenue,
6 SUM(IF(month="Apr", revenue, null)) as Apr_Revenue,
7 SUM(IF(month="May", revenue, null)) as May_Revenue,
8 SUM(IF(month="Jun", revenue, null)) as Jun_Revenue,
9 SUM(IF(month="Jul", revenue, null)) as Jul_Revenue,
10 SUM(IF(month="Aug", revenue, null)) as Aug_Revenue,
11 SUM(IF(month="Sep", revenue, null)) as Sep_Revenue,
12 SUM(IF(month="Oct", revenue, null)) as Oct_Revenue,
13 SUM(IF(month="Nov", revenue, null)) as Nov_Revenue,
14 SUM(IF(month="Dec", revenue, null)) as Dec_Revenue
15FROM Department
16GROUP BY id
也可以用 CASE WHEN THEN。
原题
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reformat-department-table
