Determine if date range is contained in another range in MySQL - mysql

Question

I'm looking for a method to do this in a "clean" way (not 3..n cross JOINS), just want to know if it's possible to do it in sql, if not I'll go for another solution.
Will use numbers instead of dates for simplification
I have n rows with n tasks and n items
task item start end
1 1 1 5
1 2 2 6
1 3 0 4
1 4 8 10
In this case I'm looking to use the min(start) max(end) of the overlapping dates so the result will be:
task item start end
1 1,2,3 0 6
1 4 8 10
Any ideas of how to resolve it in sql? is like a challenge, if can't do it this way I'll go to python.
Thank you

Answer 1

This similar to the problem I answered here, and similar data "island" problems. However, it is more complicated in your case as the identification of the "islands" will need to be calculated from more than just the record immediately prior.
It will end up looking something like this:
SET #iEnd = -1; /* init value should be something you don't expect to see */
SET #task = -1; /* init value should be something you don't expect to see */
SET #isNewIsland = 0 /* init value doesn't actually matter */;
SET #i = 0;
SELECT islandNum
, GROUP_CONCAT(item ORDER BY item) AS items
, MIN(start) AS iStart
, MAX(end) AS iEnd
FROM (
SELECT #isNewIsland := IF(#task <> task OR start > #iEnd, 1, 0)
, #task := task, item, start, end
, #i := IF(#isNewIsland = 1, #i + 1, #i) AS islandNum
, #end := IF(#isNewIsland = 1, end, GREATEST(end, #iEnd))
FROM ( /* Session(#) variables evaluation can be a bit unpredictable
the subquery helps guarantee ordering before evaluation */
SELECT task, item, start, end
FROM theTable
ORDER BY task, start, end
) AS subQ
) AS subQ 2
some are not fond of needing the separate, preceding SET statements; to avoid the need, replace ) AS subQ with
) AS subQ, (SELECT #iEnd := -1, #task := -1, #isNewIsland := 0, #i := 0) AS sInit