SQL: 重新格式化部门表 (行转列问题：Group by + 聚合函数)

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)

使用 group by 需要使用聚合函数，这里的聚合函数可以用 max 、 min 、 sum 等

select
    id,
    sum(case when month='Jan' then revenue else null end) Jan_Revenue,
    sum(case when month='Feb' then revenue else null end) Feb_Revenue,
    sum(case when month='Mar' then revenue else null end) Mar_Revenue,
    sum(case when month='Apr' then revenue else null end) Apr_Revenue,
    sum(case when month='May' then revenue else null end) May_Revenue,
    sum(case when month='Jun' then revenue else null end) Jun_Revenue,
    sum(case when month='Jul' then revenue else null end) Jul_Revenue,
    sum(case when month='Aug' then revenue else null end) Aug_Revenue,
    sum(case when month='Sep' then revenue else null end) Sep_Revenue,
    sum(case when month='Oct' then revenue else null end) Oct_Revenue,
    sum(case when month='Nov' then revenue else null end) Nov_Revenue,
    sum(case when month='Dec' then revenue else null end) Dec_Revenue
from Department
group by id

select
    id,
    max(case when month='Jan' then revenue else null end) Jan_Revenue,
    max(case when month='Feb' then revenue else null end) Feb_Revenue,
    max(case when month='Mar' then revenue else null end) Mar_Revenue,
    max(case when month='Apr' then revenue else null end) Apr_Revenue,
    max(case when month='May' then revenue else null end) May_Revenue,
    max(case when month='Jun' then revenue else null end) Jun_Revenue,
    max(case when month='Jul' then revenue else null end) Jul_Revenue,
    max(case when month='Aug' then revenue else null end) Aug_Revenue,
    max(case when month='Sep' then revenue else null end) Sep_Revenue,
    max(case when month='Oct' then revenue else null end) Oct_Revenue,
    max(case when month='Nov' then revenue else null end) Nov_Revenue,
    max(case when month='Dec' then revenue else null end) Dec_Revenue
from Department
group by id