LeetCode刷题191127

数据库：

1179

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)。

链接：https://leetcode-cn.com/problems/reformat-department-table

复习一下行转列，贴一个网友的答案

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`;

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