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

英文原文

Table: Department

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| revenue     | int     |
| month       | varchar |
+-------------+---------+
(id, month) is the primary key of this table.
The table has information about the revenue of each department per month.
The month has values in ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"].

 

Write an SQL query to reformat the table such that there is a department id column and a revenue column for each month.

Return the result table in any order.

The query result format is in the following example.

 

Example 1:

Input: 
Department table:
+------+---------+-------+
| id   | revenue | month |
+------+---------+-------+
| 1    | 8000    | Jan   |
| 2    | 9000    | Jan   |
| 3    | 10000   | Feb   |
| 1    | 7000    | Feb   |
| 1    | 6000    | Mar   |
+------+---------+-------+
Output: 
+------+-------------+-------------+-------------+-----+-------------+
| id   | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1    | 8000        | 7000        | 6000        | ... | null        |
| 2    | 9000        | null        | null        | ... | null        |
| 3    | null        | 10000       | null        | ... | null        |
+------+-------------+-------------+-------------+-----+-------------+
Explanation: The revenue from Apr to Dec is null.
Note that the result table has 13 columns (1 for the department id + 12 for the months).

中文题目

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)。

通过代码

高赞题解

一道经典的行转列。

看了很多题解，各位大佬思路都很好，但我就是没明白为什么要用到sum()或者max()。

后来看到有评论提供的关于group by原理的链接：https://blog.csdn.net/u014717572/article/details/80687042，看完之后终于有点懂了。

请大家先看完上面链接，有助于理解下面的内容。

这里分享一下我的看法（我自己的看法，仅供参考哈）

1、我先把答案写出来，

SELECT id, 
SUM(CASE WHEN month='Jan' THEN revenue END) AS Jan_Revenue,
SUM(CASE WHEN month='Feb' THEN revenue END) AS Feb_Revenue,
SUM(CASE WHEN month='Mar' THEN revenue END) AS Mar_Revenue,
SUM(CASE WHEN month='Apr' THEN revenue END) AS Apr_Revenue,
SUM(CASE WHEN month='May' THEN revenue END) AS May_Revenue,
SUM(CASE WHEN month='Jun' THEN revenue END) AS Jun_Revenue,
SUM(CASE WHEN month='Jul' THEN revenue END) AS Jul_Revenue,
SUM(CASE WHEN month='Aug' THEN revenue END) AS Aug_Revenue,
SUM(CASE WHEN month='Sep' THEN revenue END) AS Sep_Revenue,
SUM(CASE WHEN month='Oct' THEN revenue END) AS Oct_Revenue,
SUM(CASE WHEN month='Nov' THEN revenue END) AS Nov_Revenue,
SUM(CASE WHEN month='Dec' THEN revenue END) AS Dec_Revenue
FROM department
GROUP BY id
ORDER BY id;

2、讲讲group by起的作用
GROUP BY id 会使department表按照id分组，生成一张虚拟表（假想中的表）如下：
+——+———+——-+
| id | revenue | month |
+——+———+——-+
| | 8000 | Jan |
| 1 | 7000 | Feb |
| | 6000 | Mar |
+——+———+——-+
| 2 | 9000 | Jan |
+——+———+——-+
| 3 | 10000 | Feb |

在虚拟表中，所有id=1的revenue或者month数据都写在了同一个单元格中，如8000、7000、6000都是写在同一单元格内的。真正的表是不能这样写的，所以这种写法只存在于虚拟表中，帮助我们理解。

3、讲讲case when的原理
当一个单元格中有多个数据时，case when只会提取当中的第一个数据。

以CASE WHEN month=’Feb’ THEN revenue END 为例，当id=1时，它只会提取month对应单元格里的第一个数据，即Jan，它不等于Feb，所以找不到Feb对应的revenue，所以返回NULL。（可以试试把我上面答案里的sum()统统去掉，执行结果与预期不一样。错就错在当id=1时,Feb_Revenue和Mar_Revenue的值变成了NULL）

那该如何解决单元格内含多个数据的情况呢？答案就是使用聚合函数，聚合函数就用来输入多个数据，输出一个数据的。如SUM()或MAX()，而每个聚合函数的输入就是每一个多数据的单元格。

以SUM(CASE WHEN month=’Feb’ THEN revenue END) 为例，当id=1时，它提取的Jan、Feb、Mar，从中找到了符合条件的Feb，并最终返回对应的revenue的值，即7000。

统计信息

通过次数	提交次数	AC比率
32364	50841	63.7%

提交历史

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