MS Access year to date numbers by month - ms-access

Question

I have a query in an Access database which returns the results below:
MthName 2010
Jan £4.51
Feb £10.20
Mar £17.51
Apr £22.86
May £28.82
Jun £33.30
Jul £37.96
Aug £42.52
Sep £47.88
Oct £54.25
Nov £60.52
Dec £65.80
That is fine but these are year to date numbers and I would like to create a query that could give me the actual month numbers instead.
Taking the above sample the Jan number is clearly £4.51 but the Feb number is
(£10.2-£4.51)= £5.70
I have tried using something like DLOOKUP but it seems that would be really slow and wasn't working correctly.
This can be achieved very easily in Excel but I was hoping to find a query for future use.
Thanks

Answer 1

Avoid using Dlookup as much as you can.
if it is possible to add column [mntNum] with SEQUENCE number of month :
mtnNum MthName 2010
01 Jan £4.51
02 Feb £10.20
03 Mar £17.51
04 Apr £22.86
05 May £28.82
06 Jun £33.30
07 Jul £37.96
08 Aug £42.52
09 Sep £47.88
10 Oct £54.25
11 Nov £60.52
12 Dec £65.80
Query
SELECT table.mntNum, table.mntName, table.[2010], [2010]-nz((SELECT [prev].[2010] FROM [table] as [prev] where [prev].[mntNum]=[table].[mntNum]-1),0) AS JUST_THIS_MONTH
FROM [table];
UPD:
Result of this query
mntNum mntName 2010 JUST_THIS_MONTH
1 Jan 4.51 4.51
2 Feb 10.2 5.69
3 Mar 17.51 7.31
4 apr 22.86 5.35
5 may 28.82 5.96
6 jun 33.3 4.48
7 jul 37.96 4.66
8 Aug 42.52 4.56
9 Sep 47.88 5.36
10 Oct 54.25 6.37
11 Nov 60.52 6.27
12 Dec 65.8 5.27999999999999