Subject Re: [firebird-support] QUERY QUESTION setysvar 2017-03-03T20:26:32Z
Den 02.03.2017 17:36, skrev 'Stef' stef@... [firebird-support]:
> Good day,
>
> I have a question for you SQL boffins, how can I query a table to get the
> count of consecutive days found ?
>
> A Table contains rows with dates where an item(s) has been added for every
> day this item was used.
>
> i.e.
>
> itemnr dateused
>
> abc 2017/02/01
> abc 2017/02/02
> abc 2017/02/25
> abc 2017/02/25
> abc 2017/02/25
> abc 2017/02/26
> abc 2017/02/27
> abc 2017/02/28
> abc 2017/03/01
> abc 2017/03/02
>
> The result from above sample should be 6 as there is a break between
> 2017/02/02 and 2017/02/25?
Hi Stef, thanks for teaching me a new word, I've never heard 'boffin'
before!

I think your task roughly can be reformulated as:

I want to find the number of days from the most recent day of a record
without any record for the preceeding day to the most recent day for a
particular item, and add 1 to the result.

If so, this query will get you what you're asking for, although it may
take a while to return any result if the table is large:

WITH TMP(ItemNr, FirstDate, LastDate) AS
(SELECT TTFirst.ItemNr, MAX(TTFirst.DateUsed), MAX(TTLast.DateUsed)
FROM TEST_TABLE TTFirst
JOIN TEST_TABLE TTLast ON TTFirst.ItemNr = TTLast.ItemNr
WHERE NOT EXISTS(SELECT * FROM TEST_TABLE TTNot
WHERE TTFirst.ItemNr = TTNot.ItemNr
AND TTFirst.DateUsed - 1 = TTNot.DateUsed)
GROUP BY 1)
SELECT ItemNr, LastDate, LastDate - FirstDate + 1 as ConsecutiveDays
FROM TMP

So, what originally seems simple but grows complex once thinking about
how to solve it, happens to have a rather simple solution.

HTH,
Set