How to fill the gaps? - sql

Question

Assuming I have two records, both with a date and a count:
--Date-- --Count--
2011-09-20 00:00:00 5
2011-09-16 00:00:00 8
How would you select this for filling the time gaps, always taking the last previous record?
So the output would be:
--Date-- --Count--
2011-09-20 00:00:00 5
2011-09-19 00:00:00 8
2011-09-18 00:00:00 8
2011-09-17 00:00:00 8
2011-09-16 00:00:00 8
I couldn't figure out a neat solution for this, yet.
I guess this could be done with DATEDIFF, and a for-loop, but I hope this can be done easier.

Answer 1

You have 2 issues you're trying to resolve. The first issue is how to fill the gaps. The second issue is populating the Count field for those missing records.
Issue 1: This can be resolved by either using a Dates Lookup table or by creating a recursive common table expression. I would recommend creating a Dates Lookup table for this if that is an option. If you cannot create such a table, then you're going to need something like this.
WITH CTE AS (
SELECT MAX(dt) maxdate, MIN(dt) mindate
FROM yourtable
),
RecursiveCTE AS (
SELECT mindate dtfield
FROM CTE
UNION ALL
SELECT DATEADD(day, 1, dtfield)
FROM RecursiveCTE R
JOIN CTE T
ON R.dtfield < T.maxdate
)
That should create you a list of dates starting with the MIN date in your table and ending in the MAX.
Issue 2: Here is where a correlated subquery would come in handy (as much as I generally stay away from them) to get the last cnt from your original table:
SELECT r.dtfield,
(SELECT TOP 1 cnt
FROM yourtable
WHERE dt <= r.dtfield
ORDER BY dt DESC) cnt
FROM RecursiveCTE r
SQL Fiddle Demo

Answer 2

My solution goes like this.
Step 1: Have a Date table which has all the dates. - you can use many methods ex: Get a list of dates between two dates
Step 2: Do a Left outer from the date table to your result set. - which would result you with the below resultset: Call this table as "TEST_DATE_COUnt"
--Date-- --Count--
2011-09-20 00:00:00 5
2011-09-19 00:00:00 0
2011-09-18 00:00:00 0
2011-09-17 00:00:00 0
2011-09-16 00:00:00 8
Step 3: Do a Recursive query like below:
SELECT t1.date_x, t1.count_x,
(case when count_x=0 then (SELECT max(COUNT_X)
FROM TEST_DATE_COUNT r
WHERE r.DATE_X <= t1.DATE_X)
else COUNT_X
end)
cnt
FROM TEST_DATE_COUNT t1
Please let me know if this works. I tested and it worked.