SQL Server T-SQL: Get a records set based on a priority value, startdate and enddate - sql-server

Question

I am working on an time attendance system. I have to following tables:
Schedule: Contains a Name nvarchar field and a Start and End DateTime fields.
Policy: Contains Start and End DateTime fields too.
PolicySchedule (Cross Table): Contains a Priority int field beside the foreign keys.
End datetime fields are nullable which indicates open periods.
The schedule of the greatest priority will be applied and activated within its time and the policy time.
I need to get a list of the applied schedules within each policy and their activation periods start and end time knowing that shedules may be intersected. Policies are not related here ..
Example:
Schedule_____________Start_________________________End
Schedule1____________01/01/2011 00:00______________04/01/2011 00:00
Schedule2____________04/01/2011 00:00______________11/01/2011 14:00
Schedule3____________11/01/2011 14:00______________02/15/2012 00:00
Schedule2____________02/15/2012 00:00______________01/01/2013 00:00
What is the most efficient way to get the requested result ?

Answer 1

If you have lots of policies and schedules (separately) but few schedules per policy, the most straightforward way would be quite efficient:
WITH dates (policyId, changeDate) AS
(
SELECT policyId, changeDate,
ROW_NUMBER() OVER (PARTITION BY policyId ORDER BY changeDate) AS rn
FROM (
SELECT policyId, startDate AS changeDate
FROM policySchedule ps
JOIN schedule s
ON s.id = ps.scheduleId
UNION
SELECT policyId, endDate AS changeDate
FROM policySchedule ps
JOIN schedule s
ON s.id = ps.scheduleId
) q
),
ranges (startDate, endDate)AS
(
SELECT d1.policyId,
d1.changeDate,
d2.changeDate
FROM dates d1
JOIN dates d2
ON d2.policyId = d1.policyId
AND d2.rn = d1.rn + 1
)
SELECT *
FROM policy p
JOIN ranges r
ON r.policyId = p.id
CROSS APPLY
(
SELECT TOP 1 s.*
FROM policySchedules ps
JOIN schedule s
ON ps.policyId = p.id
AND s.id = ps.scheduleId
WHERE ps.startDate BETWEEN r.startDate AND r.endDate
AND ps.endDate BETWEEN r.startDate AND r.endDate
ORDER BY
ps.Priority DESC
)

Answer 2

Assuming you're using SQL Server 2005 or later, you can use an outer apply to look up the policy with the highest priority:
select *
from Schedule s
outer apply
(
select top 1 *
from PolicySchedule ps
join Policy p
on p.id = ps.policyid
where s.StartTime <= p.EndTime
and p.StartTime <= s.EndTime
order by
ps.priority desc
) pol
If there are time periods that have to overlap, you can add a where clause in the outer apply.