重新格式化部门表

地址:https://leetcode-cn.com/problems/reformat-department-table/

## 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。 
     示例：
    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)。
`解题思路`

1 case when then

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