i have a table for example:
id sales filedate
1 500 6/1/2012
2 200 6/1/2012
.
.
1 400 5/1/2012
2 175 5/1/2012
.
.
1 300 4/1/2012
2 180 4/1/2012
.
.
My table will not have any dates other than the first day of a month.
I would like to find the difference between the sales for the same id numbers for the dates 5/1/2012 and 6/1/2012 and show the following: id sale diff
How do I do this?
Note: My actual table has 24 months of sales data as columns for each date.