题目

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)。

题解

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`