SQL SERVER generating row number based on criteria - sql-server-2008

Question

I have a table which has columns UserID,DIFFTIME. when I select these columns from the table, I also want to have a derived column which is : If the DiffTime is > 20 I want to increment the count per user id.
For example if the table has:
User ID DIFF TIME
1 0
1 5
1 10
2 0
2 21
2 5
I want a result set that is something like this:
User ID DIFF TIME SESSION NUMBER
1 0 1
1 5 1
1 10 1
2 0 1
2 21 2
2 5 2
How do I accomplish this.
Ideas and suggestions are much appreciated!

Answer 1

Use this statement:
select t1.User_Id, t1.Diff_Time,
isnull(count(t2.User_Id), 0) + 1 as Session_Number
from #table t1
left join #table t2
on t1.User_Id = t2.User_Id
and t1.eventTime >= t2.eventTime
and t2.Diff_Time > 20
group by t1.User_Id, t1.Diff_Time, t1.eventTime
order by t1.User_Id, t1.eventTime
(replace #table with your actual table name)
Note: I assume that the fifth row of your table has the value 21 in the Diff_Time column, and there's a typo in the question, as #AaronBertrand pointed out in the comments

Answer 2

create table #t
(
id int,
Diff int,
SessionNumber int
)
insert into #t(id, diff)values(1, 0)
insert into #t(id, diff)values(1, 5)
insert into #t(id, diff)values(1, 10)
insert into #t(id, diff)values(2, 0)
insert into #t(id, diff)values(2, 21)
insert into #t(id, diff)values(2, 5)
Select ROW_NUMBER() over(order by Id) as RowID, * into #Temp1 from #t
Declare #diff int
Declare #RowId int
Declare #Previous int
Declare #NewValue int
DECLARE #Cur CURSOR SET #Cur = CURSOR FOR select RowId, diff from #Temp1
OPEN #Cur
FETCH NEXT FROM #Cur INTO #RowId, #diff
WHILE ##FETCH_STATUS = 0
BEGIN
if(#RowId = 1)
Begin
Update #Temp1 Set SessionNumber = 1 Where rowid = 1
Set #Previous = #Diff
Set #NewValue = 1
End
Else
Begin
if(#Diff - #Previous > 20)
Begin
Set #Previous = #Diff
Set #NewValue = #NewValue + 1
Update #Temp1 Set SessionNumber = #NewValue Where rowid = #RowId
End
else
Update #Temp1 Set SessionNumber = #NewValue Where rowid = #RowId
End
FETCH NEXT FROM #Cur INTO #RowId, #diff
END
CLOSE #Cur
DEALLOCATE #Cur
select * from #temp1
drop table #t
drop table #temp1