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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