문제

1179. Reformat Department Table

+-------------+---------+
| 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).

풀이방법

부서별 월간 수익율을 구하는 문제로 case문을 활용해 월별 Revenue의
합계를 구함~!

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