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생각보다 쉽게 풀었으나, 꼭 기억할 것이 있는 문제.
group by 함수를 쓰는 것만으로도 모두 다 집계되지 않을 수 있으니 꼭 잘 확인할 것!

1179. Reformat Department Table

Table: Department

+-------------+---------+
| Column Name | Type |
+-------------+---------+
| id | int |
| revenue | int |
| month | varchar |
+-------------+---------+
In SQL,(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"].

Reformat the table such that there is a department id column and a revenue column for each month.

풀이

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