重新格式化部门表

题目描述

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)。

答案

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

题解

(id, month) 是联合主键 故分组后，每个分组中的月份都是唯一的.聚合函数 使用MIN 或 MAX都行