LeetCode：Database 08.重新格式化部门表

要求：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的结构：

+---------------+---------+
| 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"]。

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        |
+------+-------------+-------------+-------------+-----+-------------+

分析：
1.查询结果为id，1-12月的收入，因此我们需要用到case when将收入的列名命名为1-12月的收入
2.通过id分组查询到每个id对应的1-12月收入
3.分组后查询结果每个id，只有一行，为了获得每个月的收入，我们需要使用聚合函数获得我们需要的结果，使用聚合函数sum或者max都可以

SQL语句

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