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一、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)。

来源：力扣（LeetCode）
链接：leetcode.cn/problems/re…
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二、解题

根据题意创建数据表

CREATE TABLE IF NOT EXISTS Department (
	id INT,
	revenue INT,
	MONTH VARCHAR(255)
)
INSERT INTO Department VALUES
(1, 8000, 'Jan'),
(2, 9000, 'Jan'),
(3, 10000, 'Feb'),
(1, 7000, 'Feb'),
(1, 6000, 'Mar')

思路

本题可其他的题目稍有不同，本题要设置12个月份的字段名
首先确定一点排序规则，从id上看是升序，因此只需要设置Order by id就可以了，默认是升序
然后看重复数据7000，这里可以得到需要分组，并且以id分组，这是设置group by id
最后一步设置字段名，这里字段分别是 Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue 这里可以使用sum(case),如果字段有值则填充没有值则为null

代码执行

SELECT 
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;

三、执行结果

sum(case)结果

四、总结

sum(case)组合：意思是某列属性下的属性值有则填充数据，没有则NULL case when then end: 用于计算条件列表并返回多个可能结果表达式

LeetCode#重新格式化部门表