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;