1. 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
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

• 按部门id分组，组合 case 月份 when + 求和

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

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