I'm not an ASP.NET programmer - I do my web applications in PHP - but there appear to be two ASP.NET methods to call external programs... System.Diagnostics.Process and Shell()
The basic idea is you have an external command line application - in this case exiftool - and you call (execute) it from your ASP.NET script, passing it whatever parameters you want. ASP.NET launches exiftool with those parameters, exiftool runs, and then returns the results back to ASP.NET when done.
Here's an article that appears to have relevant code examples:http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.framework.aspnet/2004-03/8427.html
I can definitely help on the specifics of how to use exiftool - that'll be half the battle. If you don't want to integrate it into your ASP.NET app, then you'll need a script to extract the data from your SQL database, and then call exiftool - basically, it'll take more or less the same knowledge as above.
Grab exiftool - there's a bunch of good examples of basic writing metadata on the site