ACCESS/SQL: Calculating the difference between rows (not dates) - sql

Question

I looked through the questions here but didn't find one that suited my case.
I'm trying to write a query that will output the difference between rows
Here is a table:
ITEM CYCLES
--------------------
itemA 5
itemA 17
itemA 20
itemA 22
itemB 26
itemB 30
itemB 37
it is actually obtained by a query, and with an order by (item, cycles)
here is what I'd like the query to give me:
itemA 12
itemA 3
itemA 2
itemB 4
itemB 7
I have absolutely no idea how to proceed in SQL. Is it even possible ?
or do I have to write a function ?
*****************************EDIT*********************************
I appologize for the lack of precision, and even some absurd mistakes. I was rushing out and wrote hastily :/
I'm analyzing item failures, and need to output cycles between failures.
ITEM column is just the item ID, and
CYCLES is the number of cycles the item had when the failure occurred.
And actually looking at it today I don't understand why I put that middle column (A,B,C...) which I don't have in my table.
And indeed, I don't need to output zero values, but there shouldn't be any anyway.
I'll try the provided solutions and will get back; thanks for the answers !

Answer 1

Here's an updated solution, based on the changes to your question. Note that you'll need to change Qry to the name of your query:
SELECT Qry.Item, Qry.Cycles - (SELECT TOP 1 Cycles FROM Qry AS Q
WHERE Qry.Item=Q.Item
AND Q.Cycles < Qry.Cycles
ORDER BY Q.Cycles DESC) AS Diff
FROM Qry
WHERE (SELECT TOP 1 Cycles FROM Qry AS Q
WHERE Qry.Item=Q.Item AND Q.Cycles < Qry.Cycles) Is Not Null
ORDER BY Qry.Item, Qry.Cycles;
This produces the following output:
Item Diff
itemA 12
itemA 3
itemA 2
itemB 4
itemB 7
I assume that the 6 in your sample output was a typo, since 30 - 26 = 4.

Answer 2

Assuming our columns are named ItemName, Letter, Num, something like the following might do it:
SELECT T1.ItemName, T1.Letter
, T1.Num, [T2].[Num]-Nz([T1].[Num],[T2].[Num]) AS Expr1
FROM Table1 AS T1
LEFT JOIN Table1 AS T2
ON (T1.ItemName = T2.ItemName
And Asc([T1].[Letter]) = Asc([T2].[Letter]) - 1 )
Where [T2].[Num] <> Nz([T1].[Num],[T2].[Num])
Note that you cannot create this using the QBE grid. You would need to create in code or in the SQL View.

Answer 3

How about:
SELECT b.Item, b.[No], (
SELECT Top 1 a.No
FROM items a
WHERE a.No > b.No
ORDER BY a.Item,a.No) AS NextNo,
[NextNo]-[No] AS Result
FROM items AS b;

Answer 4

This is much simpler if you have a fourth column with an ID:
Col0 Col1 Col2 Col3
1 itemA A 5
2 itemA B 17
3 itemA C 20
4 itemA D 22
5 itemB A 26
6 itemB B 30
7 itemB C 37
Then you can use the following query:
SELECT Tbl.Col1, Tbl.Col3 - Prev.Col3 AS Diff
FROM Tbl INNER JOIN Tbl AS Prev
ON Tbl.Col0 - 1 = Prev.Col0
AND Tbl.Col1 = Prev.Col1
To produce exactly what you requested:
Col1 Diff
itemA 12
itemA 3
itemA 2
itemB 4
itemB 7
You would need to manage the Col0 data to make sure you don't end up with gaps (ie, Autonumber field would be a bad idea).
All of the above having been said, if you let us know what you are really trying to accomplish I'm thinking there is probably a better way to do what you want.