How can I contstruct this T-SQL query involving missing date ranges? - sql

Question

I'll try to keep the specific details of my problem out of this question and focus only on the pertinent issues.
Lets say I have an Assets table with a primary key of AssetID.
I have another table called ProcessedDates with primary key PID and with additional columns AssetID, StartDate, EndDate.
I want to run a process for a list of assets between a start date and end date. Before I can run this process, I need to know which assets and which date ranges have already been processed.
For example, there are 2 entries in ProcessedDates:
AssetID StartDate EndDate
--------------------------
Asset1 Day4 day7
Asset1 Day10 Day12
I want to process Asset1 between day2 and day11. I don't need to waste time by processing on days that have already been done so in this example, I will only process asset1 from day2 to day3 and from day8 to day 9.
So what I need is a query that returns the gaps in the date ranges. In this case, the result set will be 2 lines:
AssetID StartDate EndDate
--------------------------
Asset1 day2 day3
Asset1 day8 day9
In my actual requirement I have many assetIDs. The ProcessedDates table may have multiple entries for each asset or none at all and each asset does not necessarily have the same processed dates as any other asset.
declare #StartDate date, #EndDate date (assume these are given)
--get distinct assets
select distinct AssetIDs from (some query) into #Assets
--get the already processed date ranges
select p.AssetID, p.StartDate, p.EndDate
from ProcessedDates p inner join #Assets a on p.AssetID = a.AssetID
where p.StartDate between #StartDate and #EndDate
or p.EndDate between #StartDate and #EndDate
From here I have no clue how to proceed. How do I get it to return AssetID, StartDate, EndDate for all the gaps in between?

Answer 1

Something like this:
declare #StartDate date = '2015-01-01', #EndDate date = '2015-05-05'
declare #Assets table (AssetID varchar(50), StartDate date, EndDate date)
declare #AssetTypes table (AssetID varchar(50))
insert into #AssetTypes values
('Asset1'),
('Asset2')
insert into #Assets values
('Asset1', '2014-12-10', '2014-12-31'), -- Ignored
('Asset1', '2015-02-02', '2015-03-02'),
('Asset1', '2015-03-05', '2015-05-01'),
('Asset1', '2015-06-01', '2015-06-06') -- Ignored
;WITH Base AS (
SELECT AT.AssetID
, CASE WHEN A.AssetID IS NULL THEN 1 ELSE 0 END EmptyAsset
, A.StartDate
, A.EndDate
, ROW_NUMBER() OVER (PARTITION BY AT.AssetID ORDER BY StartDate) RN
FROM #AssetTypes AT
LEFT JOIN #Assets A ON A.AssetID = AT.AssetID
WHERE A.AssetID IS NULL -- case of totally missing asset
OR (StartDate <= #EndDate AND EndDate >= #StartDate)
)
-- first missing range, before the first row
SELECT AssetID, #StartDate StartDate, DATEADD(dd, -1, StartDate) EndDate
FROM Base
WHERE RN = 1 AND StartDate > #StartDate
UNION ALL
-- each row joined with the next one
SELECT B1.AssetID, DATEADD(dd, 1, B1.EndDate), ISNULL(DATEADD(dd, -1, B2.StartDate), #EndDate)
FROM Base B1
LEFT JOIN Base B2 ON B2.AssetID = B1.AssetID AND B2.RN = B1.RN + 1
WHERE B1.EmptyAsset = 0
AND (B2.AssetID IS NULL -- Last row case
OR DATEADD(dd, 1, B1.EndDate) < B2.StartDate) -- Other rows case
AND B1.EndDate < #EndDate -- If the range ends after #EndDate, nothing to do
UNION ALL
-- case of totally missing asset
SELECT AssetID, #StartDate, #EndDate
FROM Base
WHERE EmptyAsset = 1
The main idea is that each row is joined with the next one. A new range is generated (if necessary) between the EndDate + 1 and the StartDate - 1. There is a special handling for the last row (B2.AssetID IS NULL and ISNULL(... #EndDate)). The first SELECT generated a row before the first range, and the last select is for the special case of no ranges present for an asset.
As I've written in the comments, it gets ugly quite quickly.

Answer 2

Here's an simple version to get the result you want. I use integer as date, and assume the min date is 0 and the max date is 999.
--DDL
create table Assets (AssetID integer, StartDate integer, EndDate integer);
insert into Assets values
(1,4,7),
(1,10,12),
(1,15,17),
(2,5,7),
(2,9,10);
with temp as(
select a1.AssetId,
a1.enddate+1 as StartDate,
coalesce(min(a2.startdate) - 1,999) as EndDate
from Assets a1
left join Assets a2
on a1.assetid = a2.assetid
and a1.enddate < a2.startdate
group by a1.assetid,a1.enddate
union all
select a.assetid,0,min(startdate) -1
from Assets a
group by a.assetid
)
select AssetId,
case when StartDate<2 then 2 else StartDate end as StartDate,
case when EndDate>11 then 11 else EndDate end as EndDate
from temp
where StartDate<=11 and EndDate>=2
order by AssetId,StartDate
The temp table can get the missing ranges. Then filter the match ranges between Day2 and Day11, will get the result that you want.
AssetId StartDate EndDate
1 2 3
1 8 9
2 2 4
2 8 8
2 11 11
Here's the SqlFiddle Demo