dynamic interval creation in SQL - sql

Question

I have the following problem, that I would like to solve with transact-sql.
I have something like this
Start | End | Item
1 | 5 | A
3 | 8 | B
and I want to create something like
Start | End | Item-Combination
1 | 2 | A
3 | 5 | A-B
6 | 8 | B
For the Item-Combination concatenation I already thought of using the FOR XML statement. But in order to create the different new intervals... I really don't know how to approach it. Any idea?
Thanks.

Answer 1

I had a very similar problem with some computer usage data. I had session data indicating login/logout times. I wanted to find the times (hour of day per day of week) that were the most in demand, that is, the hours where the most users were logged in. I ended up solving the problem client-side using hash tables. For each session, I would increment the bucket for a particular location corresponding to the day of week and hour of day for each day/hour for which the session was active. After examining all sessions the hash table values show the number of logins during each hour for each day of the week.
I think you could do something similar, keeping track of each item seen for each start/end value. You could then reconstruct the table by collapsing adjacent entries that have the same item combination.
And, no, I could not think of a way to solve my problem with SQL either.

Answer 2

This is a fairly typical range-finding problem, with the concatenation thrown in. Not sure if the following fits exactly, but it's a starting point. (Cursors are usually best avoided except in the small set of cases where they are faster than set-based solutions, so before the cursor haters get on me please note I use a cursor here on purpose because this smells to me like a cursor-friendly problem -- I typically avoid them.)
So if I create data like this:
CREATE TABLE [dbo].[sourceValues](
[Start] [int] NOT NULL,
[End] [int] NOT NULL,
[Item] [varchar](100) NOT NULL
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[sourceValues] WITH CHECK ADD CONSTRAINT [End_after_Start] CHECK (([End]>[Start]))
GO
ALTER TABLE [dbo].[sourceValues] CHECK CONSTRAINT [End_after_Start]
GO
declare #i int; set #i = 0;
declare #start int;
declare #end int;
declare #item varchar(100);
while #i < 1000
begin
set #start = ABS( CHECKSUM( newid () ) % 100 ) + 1 ; -- "random" int
set #end = #start + ( ABS( CHECKSUM( newid () ) % 10 ) ) + 2; -- bigger random int
set #item = char( ( ABS( CHECKSUM( newid() ) ) % 5 ) + 65 ); -- random letter A-E
print #start; print #end; print #item;
insert into sourceValues( Start, [End], Item) values ( #start , #end, #item );
set #i += 1;
end
Then I can treat the problem like this: each "Start" AND each "End" value represents a change in the collection of current Items, either adding one or removing one, at a certain time. In the code below I alias that notion as "event," meaning an Add or Remove. Each start or end is like a time, so I use the term "tick." If I make a collection of all the events, ordered by event time (Start AND End), I can iterate through it while keeping a running tally in an in-memory table of all the Items that are in play. Each time the tick value changes, I take a snapshot of that tally:
declare #tick int;
declare #lastTick int;
declare #event varchar(100);
declare #item varchar(100);
declare #concatList varchar(max);
declare #currentItemsList table ( Item varchar(100) );
create table #result ( Start int, [End] int, Items varchar(max) );
declare eventsCursor CURSOR FAST_FORWARD for
select tick, [event], item from (
select start as tick, 'Add' as [event], item from sourceValues as adds
union all
select [end] as tick, 'Remove' as [event], item from sourceValues as removes
) as [events]
order by tick
set #lastTick = 1
open eventsCursor
fetch next from eventsCursor into #tick, #event, #item
while ##FETCH_STATUS = 0
BEGIN
if #tick != #lastTick
begin
set #concatList = ''
select #concatList = #concatlist + case when len( #concatlist ) > 0 then '-' else '' end + Item
from #currentItemsList
insert into #result ( Start, [End], Items ) values ( #lastTick, #tick, #concatList )
end
if #event = 'Add' insert into #currentItemsList ( Item ) values ( #item );
else if #event = 'Remove' delete top ( 1 ) from #currentItemsList where Item = #item;
set #lastTick = #tick;
fetch next from eventsCursor into #tick, #event, #item;
END
close eventsCursor
deallocate eventsCursor
select * from #result order by start
drop table #result
Using a cursor for this special case allows just one "pass" through the data, like a running totals problem. Itzik Ben-Gan has some great examples of this in his SQL 2005 books.

Answer 3

Thanks a lot for all the answers, for the moment I have found a way of doing it. SInce I'm dealing with a datawarehouse, and I have a Time dimension, I could do some joins with Time dimension in the style"inner join DimTime t on t.date between f.start_date and end_date".
It's not very good from the performance point of view, but it seems it's working for me.
I'll give a try to onupdatecascade implementation, to see which suits better for me.

Answer 4

This will exactly emulates and solves the mentioned problem:
-- prepare problem, it can have many rows with overlapping ranges
declare #range table
(
Item char(1) primary key,
[Start] int,
[End] int
)
insert #range select 'A', 1, 5
insert #range select 'B', 3, 8
-- unroll the ranges into helper table
declare #usage table
(
Item char(1),
Number int
)
declare
#Start int,
#End int,
#Item char(1)
declare table_cur cursor local forward_only read_only for
select [Start], [End], Item from #range
open table_cur
fetch next from table_cur into #Start, #End, #Item
while ##fetch_status = 0
begin
with
Num(Pos) as -- generate numbers used
(
select cast(#Start as int)
union all
select cast(Pos + 1 as int) from Num where Pos < #End
)
insert
#usage
select
#Item,
Pos
from
Num
option (maxrecursion 0) -- just in case more than 100
fetch next from table_cur into #Start, #End, #Item
end
close table_cur
deallocate table_cur
-- compile overlaps
;
with
overlaps as
(
select
Number,
(
select
Item + '-'
from
#usage as i
where
o.Number = i.Number
for xml path('')
)
as Items
from
#usage as o
group by
Number
)
select
min(Number) as [Start],
max(Number) as [End],
left(Items, len(Items) - 1) as Items -- beautify
from
overlaps
group by
Items