If your priority is speed of selects, the following approach allows for extremely fast selects. Instead of storing a period, you want to store two events (period start and period end). Change column is 1 when the period begins, and is -1 when the period ends. If more than one event occurs on the same day, they must have different EventNumberPerDay. RunningTotal is the number of open peroids after the event has happened:
CREATE TABLE dbo.Events(
PeriodId INT NOT NULL,
Change SMALLINT NOT NULL,
ChangeDate DATE NOT NULL,
EventNumberPerDay INT NOT NULL,
RunningTotal INT NOT NULL);
GO
INSERT dbo.Events
( PeriodId ,
Change ,
ChangeDate ,
EventNumberPerDay,
RunningTotal
)
-- first period begins
VALUES ( 1 , 1, '20120801', 1, 1),
-- second period begins on the same day
(2, 1, '20120801', 2, 2),
-- third period begins
(3,1,'20120803',1, 3),
-- second period ends on the same day
(2,-1,'20120803',2, 2),
-- fourth period begins
(4,1,'20120804',1,3),
-- fourth period ends
(4,-1,'20120805',1,2),
-- first period ends
(1, -1, '20120808',1, 1),
-- third period ends
(3, -1, '20120809',1, 0);
GO
Also you need a calendar table:
CREATE TABLE dbo.Calendar([Date] DATE NOT NULL);
GO
INSERT INTO dbo.Calendar([Date])
VALUES('20120801'),
('20120802'),
('20120803'),
('20120804'),
('20120805'),
('20120806'),
('20120807'),
('20120808'),
('20120809'),
('20120810'),
('20120811');
Once that is accomplished, then your select is very simple and very fast:
SELECT [Date] ,
coalesce(RunningTotal, 0) AS NumOpenIntervals
FROM dbo.Calendar
OUTER APPLY ( SELECT TOP ( 1 )
RunningTotal
FROM dbo.Events
WHERE ChangeDate <= [Date]
ORDER BY ChangeDate DESC, EventNumberPerDay DESC
) AS t
ORDER BY [Date]
Of course, this query is only correct if the data in Events table is valid. We can use constraints to ensure 100% data integrity. I can explain how if you are interested.
Another alternative is to just load your raw data, your periods, into a client application - your problem is absolutely trivial in C++/C#/Java.
Yet another approach is to use an RDBMS with fast cursors such as Oracle - that will allow you to just write a simple cursor and enjoy good performance, but still not always as good as my first solution.
I think you want (tested at SQL-Fiddle-1):
SELECT the_id = dd.the_id,
the_day = ct.the_day,
the_count = COALESCE(SUM(dt.the_count), 0)
FROM
calendartable AS ct
CROSS JOIN
( SELECT DISTINCT the_id
FROM datatable
WHERE the_date BETWEEN @mindate
AND @maxdate
) AS dd
LEFT JOIN
datatable AS dt
ON dt.the_date = ct.the_day
AND dt.the_id = dd.the_id
WHERE
ct.the_day BETWEEN @mindate
AND @maxdate
GROUP BY
dd.the_id, ct.the_day
ORDER BY
the_id, the_day ;
It can also be written like this (SQL-Fiddle-2):
SELECT the_id = dd.the_id,
the_day = ct.the_day,
the_count = COALESCE(
( SELECT SUM(dt.the_count)
FROM datatable AS dt
WHERE dt.the_date = ct.the_day
AND dt.the_id = dd.the_id
)
, 0)
FROM
calendartable AS ct
CROSS JOIN
( SELECT DISTINCT the_id
FROM datatable
WHERE the_date BETWEEN @mindate
AND @maxdate
) AS dd
WHERE
ct.the_day BETWEEN @mindate
AND @maxdate
ORDER BY
the_id, the_day ;
Best Answer