mysql case 使用

+---------------+---------+
| 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)。

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

题目转自：https://leetcode-cn.com/problems/reformat-department-table/