leetcode1179-重新格式化部门表

leetcode1179. 重新格式化部门表

类型:数据库

1.问题描述

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)。

来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/reformat-department-table
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2.解题思路

对于有限的列数据,可以通过 as XXX 来创建列。来达到一个行列转换的效果。

# Write your MySQL query statement below
SELECT 
    id
    , sum(case `month` when 'Jan' then revenue else null end) as Jan_Revenue
    , sum(case `month` when 'Feb' then revenue else null end) as Feb_Revenue
    , sum(case `month` when 'Mar' then revenue else null end) as Mar_Revenue
    , sum(case `month` when 'Apr' then revenue else null end) as Apr_Revenue
    , sum(case `month` when 'May' then revenue else null end) as May_Revenue
    , sum(case `month` when 'Jun' then revenue else null end) as Jun_Revenue
    , sum(case `month` when 'Jul' then revenue else null end) as Jul_Revenue
    , sum(case `month` when 'Aug' then revenue else null end) as Aug_Revenue
    , sum(case `month` when 'Sep' then revenue else null end) as Sep_Revenue
    , sum(case `month` when 'Oct' then revenue else null end) as Oct_Revenue
    , sum(case `month` when 'Nov' then revenue else null end) as Nov_Revenue
    , sum(case `month` when 'Dec' then revenue else null end) as Dec_Revenue
FROM Department group by id;

Author: Juntech
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