SQL question, query, procedure or function

Question

This is the description.

There is a table that lists the time intervals required to perform a specific task:

id	start	Finish
1	07:00:00	11:00:00
2	12:30:00	17:00:00
3	11:30:00	14:00:00
4	09:30:00	11:30:00
5	10:00:00	15:00:00
6	09:00:00	13:30:00
7	07:00:00	11:30:00

These tasks cannot be completed at the same time. I need to find out the minimum number of days required to complete all tasks.

In this table, I can spend at least 5 days to complete the task.

day 1 => task 1 and task 2
day 2 => task 4 and task 3
day 3 => task 5
day 4 => task 6
day 5 => task 7

So the result should be 5. I hope it's clear, thank you in advance!

Answer 1

SELECT COUNT(*)
FROM test t1
JOIN test t2 ON 
-- these 2 conditions checks for overlapping
                t1.start < t2.finish
            AND t2.start < t1.finish        
-- this condition simply decreases the amount of combined rows
-- to be examined, it does not effect the output
            AND t1.id <= t2.id                
GROUP BY t1.id
-- retrieve only maximal value
ORDER BY 1 DESC LIMIT 1

You must simply calculate the maximum number of simultaneous tasks. It is enough to check the start time of each task.

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=ee3034a307c32212db365ba06f654f9f