2019.12.27 LeetCode 从零单刷个人笔记整理（持续更新）
github：https://github.com/ChopinXBP/LeetCode-Babel
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)。
#行转列问题
SELECT id,
MAX(IF(`month` = 'Jan', revenue, NULL)) AS 'Jan_Revenue',
MAX(IF(`month` = 'Feb', revenue, NULL)) AS 'Feb_Revenue',
MAX(IF(`month` = 'Mar', revenue, NULL)) AS 'Mar_Revenue',
MAX(IF(`month` = 'Apr', revenue, NULL)) AS 'Apr_Revenue',
MAX(IF(`month` = 'May', revenue, NULL)) AS 'May_Revenue',
MAX(IF(`month` = 'Jun', revenue, NULL)) AS 'Jun_Revenue',
MAX(IF(`month` = 'Jul', revenue, NULL)) AS 'Jul_Revenue',
MAX(IF(`month` = 'Aug', revenue, NULL)) AS 'Aug_Revenue',
MAX(IF(`month` = 'Sep', revenue, NULL)) AS 'Sep_Revenue',
MAX(IF(`month` = 'Oct', revenue, NULL)) AS 'Oct_Revenue',
MAX(IF(`month` = 'Nov', revenue, NULL)) AS 'Nov_Revenue',
MAX(IF(`month` = 'Dec', revenue, NULL)) AS 'Dec_Revenue'
FROM Department
GROUP BY id;
#Coding一小时，Copying一秒钟。留个言点个赞呗，谢谢你#
