SQL Get the gaps in dateranges when a list of ranges is provided - sql

Question

I'm currently looking for a SQL solution for the following problem:
SQLFiddle as guidance:
I have a list of not-nullable startdates and nullable enddates. Based on this list I need the total gap time between a given start and enddate.
Based on the SQLFiddle
If I would only have situation 1 in my database the result should be 2 days.
If I would have situation 2 and 3 in my database the result should be 1 day.
I have been pondering this for a couple of days now... any help would be much appreciated!
Regards,
Kyor
Notes: I'm running SQL 2012 ( should any special new features be required )

Answer 1

The best solution will be to create 'Dates' table and start from there, otherwise solution will be unmaintainable. For each date in specified range you can check whether it is covered by ranges in 'dateranges' table and get a count of dates that are not.
Something like this:
SELECT COUNT(*)
FROM
Dates d
WHERE
d.Date BETWEEN #start AND #end
AND NOT EXISTS
(SELECT *
FROM dateranges r
WHERE d.date BETWEEN r.startdate and ISNULL(r.enddate, d.date)
)

Answer 2

CREATE TABLE Dates (
dt DATETIME NOT NULL PRIMARY KEY);
INSERT INTO Dates VALUES('20081204');
INSERT INTO Dates VALUES('20081205');
INSERT INTO Dates VALUES('20090608');
INSERT INTO Dates VALUES('20090609');
-- missing ranges
SELECT DATEADD(DAY, 1, prev) AS start_gap,
DATEADD(DAY, -1, next) AS end_gap,
DATEDIFF(MONTH, DATEADD(DAY, 1, prev),
DATEADD(DAY, -1, next)) AS month_diff
FROM (
SELECT dt AS prev,
(SELECT MIN(dt)
FROM Dates AS B
WHERE B.dt > A.dt) AS next
FROM Dates AS A) AS T
WHERE DATEDIFF(DAY, prev, next) > 1;
-- existing ranges
SELECT MIN(dt) AS start_range,
MAX(dt) AS end_range
FROM (
SELECT dt,
DATEDIFF(DAY, ROW_NUMBER() OVER(ORDER BY dt), dt) AS grp
FROM Dates) AS D
GROUP BY grp;
DROP TABLE Dates;