selecting max date in range, excluding multiple other date ranges - sql

Question

my first time posting.
I have a tricky task of finding the latest date within a range, but excluding multiple other date ranges. I have code that does work, but it seems awfully taxing.
I am selecting the MAX(Date) within a range. However, I have a table, bfShow, where each show has its own date-range (stored as DateStart and DateEnd). So I need the MAX(Date) within the range which does NOT have a show on that date (there may be 0 to 99 shows overlapping my date-range).
Note: I have dbo.fnSeqDates which works great (found via Google) and returns all dates within a range - makes for very fast filling in 6/1/12, 6/2/12, 6/3/12...6/30/12, etc.
What I'm doing (below) is creating a table with all the dates (within range) in it, then find all the Shows within that range (#ShowIDs) and iterate through those shows, one at a time, deleting all those dates (from #DateRange). Ultimately, #DateRange is left with only "empty" dates. Thus, the MAX(Date) remaining in #DateRange is my last date in the month without a show.
Again, my code below does work, but there's got to be a better way. Thoughts?
Thank you,
Todd
CREATE procedure spLastEmptyDate
#DateStart date
, #DateEnd date
as
begin
-- VARS...
declare #ShowID int
declare #EmptyDate date
-- TEMP TABLE...
create table #DateRange(dDate date)
create table #ShowIDs(ShowID int)
-- LOAD ALL DATES IN RANGE (THIS MONTH-ISH)...
insert into #DateRange(dDate)
select SeqDate
from dbo.fnSeqDates(#DateStart, #DateEnd)
-- LOAD ALL SHOW IDs IN RANGE (THIS MONTH-IS)...
insert into #ShowIDs(ShowID)
select s.ShowID
from bfShow s
where s.DateStart = #DateStart
-- PRIME SHOW ID...
set #ShowID = 0
select #ShowID = min(ShowID)
from #ShowIDs
-- RUN THRU ALL, REMOVING DATES AS WE GO...
while (#ShowID > 0)
begin
-- REMOVE FROM TEMP...
delete DR
from #DateRange DR
, bfShow s
where DR.dDate between s.DateStart and s.DateEnd
and s.ShowID = #ShowID
-- DROP THAT ONE FROM TEMP...
delete from #ShowIDs
where ShowID = #ShowID
-- GET NEXT ID...
set #ShowID = 0
select #ShowID = min(ShowID)
from #ShowIDs
end
-- GET LAST EMPTY SPOT...
select #EmptyDate = max(dDate)
from #DateRange
-- CLEAN UP...
drop table #DateRange
drop table #ShowIDs
-- RETURN DATA...
select #EmptyDate as LastEmptyDateInRange
end

Answer 1

Let us know what version of SQL Server you're on because that will help determine your options, but you should be able to use the BETWEEN operator in a JOIN between the fnSeqDates function (it's a table-valued function, so you can join to it directly rather than inserting them into a temp table) and the bfShow tables:
SELECT TOP 1 tDate.SeqDate
FROM dbo.fnSeqDates('6/1/2012', '6/30/2012') tDate
LEFT JOIN bfShow tShow
ON tDate.SeqDate BETWEEN tShow.DateStart AND tShow.DateEnd
WHERE tShow.ShowID IS NULL -- no matches found
ORDER BY tDate.SeqDate DESC -- to pull the most recent date

Answer 2

Okay, I thought I'd re-phrase the question, and try to expose some edge cases. I'm not using your function at all. If this isn't right, can you give an example where it fails?
create table bfShow (
DateStart date,
DateEnd date
)
go
CREATE procedure spLastEmptyDate
#DateStart date
, #DateEnd date
as
--Return #DateEnd, or, if that is within a show, find the contiguous
--region of shows covering it, and select the day before that
;with ShowsCovering as (
select DateStart,DateEnd from bfShow where DateStart <= #DateEnd and DateEnd >= #DateEnd
union all
select s1.DateStart,s2.DateEnd
from
bfShow s1
inner join
ShowsCovering s2
on
s1.DateStart < s2.DateStart and
(
--This join would be helped by an indexed computed column on bfShow, either Start-1 or End+1
s1.DateEnd >= s2.DateStart or
s1.DateEnd = DATEADD(day,-1,s2.DateStart)
)
where
s2.DateStart > #DateStart
), Earliest as (
select MIN(DateStart) as MinDate from ShowsCovering
)
--1) If there are no rows, the answer is #DateEnd
--2) If there are rows, and the MIN(DateStart) = #DateStart, then no day exists
--3) Otherwise, the answer is MIN(DateStart)-1
, Answer as (
select #DateEnd as Result where exists(select * from Earliest where MinDate is null)
union all
select DATEADD(day,-1,MinDate) from Earliest where MinDate > #DateStart
)
select Result from Answer
go
insert into bfShow(DateStart,DateEnd)
values ('20120601','20120612'),
('20120619','20120630')
go
exec spLastEmptyDate '20120601','20120625'
--Result = 2012-06-18
go
exec spLastEmptyDate '20120525','20120625'
--Result = 2012-06-18
go
exec spLastEmptyDate '20120601','20120705'
--Result = 2012-07-05
go
insert into bfShow(DateStart,DateEnd)
values ('20120613','20120618')
go
exec spLastEmptyDate '20120601','20120625'
--Result - no rows
By the way, in your current solution, these lines:
drop table #DateRange
drop table #ShowIDs
Are unnecessary. Temp tables created within a stored procedure are automatically dropped when the stored procedure exits. So you can avoid the little dance at the end and make the last line just select max(dDate) as LastEmptyDateInRange from #DateRange, if you want to continue using your solution.