Yeah, it uses SQLite under the hood. SQLite has a mechanism it calls "virtual tables" which I used to implement an adapter that lets it see Excel tables as database tables. I also implemented hash indexes in the adapter so joins and lookups are crazy fast (searches by range not so much though). SQLite is very cool, it also lets developer hook additional functions into it, which is how I exposed some .NET functions and they can be used from SQL (replacex=Regex.Replace, ismatch=Regex.IsMatch, format=String.Format, etc...)