LeetCode-1179. 重新格式化部门表( 627. Swap Salary)。知识点：UPDATE and CASE...WHEN 和 if

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)。

题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reformat-department-table
审题：将竖着的表查询为分布表。

思考：SELCET 后面是月份。然后按照id排序。

将查询结果分布在其中。

解题：

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
order by id

使用if的方式：

SELECT 
    id, 
    sum( if( month = 'Jan', revenue, null ) ) AS Jan_Revenue,
    sum( if( month = 'Feb', revenue, null ) ) AS Feb_Revenue,
    sum( if( month = 'Mar', revenue, null ) ) AS Mar_Revenue,
    sum( if( month = 'Apr', revenue, null ) ) AS Apr_Revenue,
    sum( if( month = 'May', revenue, null ) ) AS May_Revenue,
    sum( if( month = 'Jun', revenue, null ) ) AS Jun_Revenue,
    sum( if( month = 'Jul', revenue, null ) ) AS Jul_Revenue,
    sum( if( month = 'Aug', revenue, null ) ) AS Aug_Revenue,
    sum( if( month = 'Sep', revenue, null ) ) AS Sep_Revenue,
    sum( if( month = 'Oct', revenue, null ) ) AS Oct_Revenue,
    sum( if( month = 'Nov', revenue, null ) ) AS Nov_Revenue,
    sum( if( month = 'Dec', revenue, null ) ) AS Dec_Revenue
FROM 
    Department
GROUP BY 
    id;