SQL Calculate (time) gap between occurrences in a log - sql-server

Question

I have tables that record when certain items were sent or returned to a particular location, and I want to work out the intervals between each time a particular item is returned.
Sample data:
Item ReturnDate:
Item1, 20120101
Item1, 20120201
Item1, 20120301
Item2, 20120401
Item2, 20120601
So in this case, we can see that the there was a month gap until Item 1 was returned the first time, and another month before it was returned the second time. Item 2 came back after 2 months.
My starting point is:
Select r1.Item, r1.ReturnDate, r2.Item, r2.ReturnDate, DateDiff(m, r1.ReturnDate, r2.ReturnDate)
from Returns r1
inner join Returns r2 on r2.VehicleNo = r1.VehicleNo
However, in the this sample, each item is compared to every other instance where it has been returned - and not just the next one. So I need to limit this query so it will only compare adjacent returns.
One solution is to tag each return with an count (of the number of times that item has been returned):
Item ReturnDate, ReturnNo:
Item1, 20120101, 1
Item1, 20120201, 2
Item1, 20120301, 3
Item2, 20120401, 1
Item2, 20120601, 2
This would enable me to use the following T-SQL (or similar):
Select r1.Item, r1.ReturnDate, r2.Item, r2.ReturnDate, DateDiff(m, r1.ReturnDate, r2.ReturnDate)
from Returns r1
inner join Returns r2 on r2.VehicleNo = r1.VehicleNo
and (r1.ReturnNo + 1 = r2.ReturnNo)
My first question is whether the is a sensible/optimal approach or whether there is a better approach?
Secondly, what is the easiest/slickest means of calculating the ReturnNo?

Answer 1

If you are using SQL Server 2005+, use ROW_NUMBER() to do exactly what you want:
WITH RankedReturn AS
(
SELECT Item, ReturnDate,
ROW_NUMBER() OVER (PARTITION BY Item ORDER BY ReturnDate DESC) AS ReturnNo
FROM Returns
)
SELECT * FROM RankedReturn
Obviously, now that you have your CTE you can put whatever you need in the outer SELECT. I would use an OUTER APPLY for this:
WITH RankedReturn AS
(
SELECT Item, ReturnDate,
ROW_NUMBER() OVER (PARTITION BY Item ORDER BY ReturnDate DESC) AS ReturnNo
FROM Returns
)
SELECT rOuter.Item, rOuter.ReturnDate, DATEDIFF(month, prev.PrevDate, ReturnDate) AS Months
FROM RankedReturn rOuter
OUTER APPLY
(
SELECT ReturnDate AS PrevDate
FROM RankedReturn rInner
WHERE rOuter.Item = rInner.Item AND rOuter.ReturnNo = rInner.ReturnNo - 1
) prev
Oops, and the SQL Fiddle is here.
Edited because the month difference calculation was backwards; fixed now

Answer 2

Easiest way of calculating the ReturnNo would be to use OVER:
SELECT [Item], [ReturnDate],
ROW_NUMBER() OVER (PARTITION BY [Item] ORDER BY [ReturnDate]) AS ReturnNumber
FROM Returns
http://sqlfiddle.com/#!3/e18ad/1/0
You could also attempt to make use of the techniques for calculating a running total to work out the difference between two rows.

Answer 3

This is how I would do it:
select itemNo,
dt,
DATEDIFF(day, previousDt, dt) as daysSince
from (select itemNo,
dt,
(select top 1 dt from testTable where itemNo = outerTbl.itemNo and dt < outerTbl.dt order by dt desc) as previousDt
from testTable as outerTbl
) as x
... and here's a bit of setup code for anybody else testing a solution to this
create table testTable(
itemNo nvarchar(20),
dt datetime)
go
insert into testTable values('Item1', '2012-01-01');
insert into testTable values('Item1', '2012-02-01');
insert into testTable values('Item1', '2012-03-01');
insert into testTable values('Item2', '2012-04-01');
insert into testTable values('Item2', '2012-05-01');
go