LeetCode 1179: Reformat Department Table (MySQL) [closed] - mysql

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This is a LeetCode question for interviews.
What would be the most efficient way (time/space complexity) to write this MySQL query? Does the coding style follow the MySQL variable naming conventions?
Problem
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.
The query result format is in the following example:
Department table:
+------+---------+-------+
| id | revenue | month |
+------+---------+-------+
| 1 | 8000 | Jan |
| 2 | 9000 | Jan |
| 3 | 10000 | Feb |
| 1 | 7000 | Feb |
| 1 | 6000 | Mar |
+------+---------+-------+
Result table:
+------+-------------+-------------+-------------+-----+-------------+
| id | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1 | 8000 | 7000 | 6000 | ... | null |
| 2 | 9000 | null | null | ... | null |
| 3 | null | 10000 | null | ... | null |
+------+-------------+-------------+-------------+-----+-------------+
Note that the result table has 13 columns (1 for the department id + 12 for the months).
Create table If Not Exists Department (id int, revenue int, month varchar(5))
Truncate table Department
insert into Department (id, revenue, month) values ('1', '8000', 'Jan')
insert into Department (id, revenue, month) values ('2', '9000', 'Jan')
insert into Department (id, revenue, month) values ('3', '10000', 'Feb')
insert into Department (id, revenue, month) values ('1', '7000', 'Feb')
insert into Department (id, revenue, month) values ('1', '6000', 'Mar')
Attempt
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;
Reference
1179. Reformat Department Table

Answer 1

I would probably use MAX as the aggregate function, not SUM, assuming a given id and month would only have one record for revenue. That being said, if you want to use SUM, then your CASE expressions should have an else condition with zero revenue:
SELECT
id,
SUM(CASE WHEN month = 'Jan' THEN revenue ELSE 0 END) AS Jan_Revenue,
SUM(CASE WHEN month = 'Feb' THEN revenue ELSE 0 END) AS Feb_Revenue,
SUM(CASE WHEN month = 'Mar' THEN revenue ELSE 0 END) AS Mar_Revenue,
SUM(CASE WHEN month = 'Apr' THEN revenue ELSE 0 END) AS Apr_Revenue,
SUM(CASE WHEN month = 'May' THEN revenue ELSE 0 END) AS May_Revenue,
SUM(CASE WHEN month = 'Jun' THEN revenue ELSE 0 END) AS Jun_Revenue,
SUM(CASE WHEN month = 'Jul' THEN revenue ELSE 0 END) AS Jul_Revenue,
SUM(CASE WHEN month = 'Aug' THEN revenue ELSE 0 END) AS Aug_Revenue,
SUM(CASE WHEN month = 'Sep' THEN revenue ELSE 0 END) AS Sep_Revenue,
SUM(CASE WHEN month = 'Oct' THEN revenue ELSE 0 END) AS Oct_Revenue,
SUM(CASE WHEN month = 'Nov' THEN revenue ELSE 0 END) AS Nov_Revenue,
SUM(CASE WHEN month = 'Dec' THEN revenue ELSE 0 END) AS Dec_Revenue
FROM department
GROUP BY
id
ORDER BY
id;