I don't know about ur data model, sorry in advance if i repeat something which you have already tried.
You need to design your data model such a way it looks like (IF I UNDERSTOOD UR BUSINESS LOGIC)
Prod Id DelDtae SalDate
A 01/01/2012 02/15/2012
A 01/05/2012 04/10/2012
Above is required if you have primary key as Product Id, if you have diff primary key then use that.
if you have primary key is Product Id, then you can achieve by using MAX, MIN functions
Thanks for your idea. However, this is not exactly what I need. I need a system that checks if an apparent sale of my product A as reported by a retailer actually could have taken place. I want realize this by looking up if up to 90 days earlier I actually supplied this product A. And this can not be solved by simply using MIN or MAX since sometimes I want to analyze data spanning many, many months.