While its interesting to theorize about databases and AGPL, I am rather sure there hasn't been any actually case or a legal reason to think that the database would become a derivative work when used together with a webserver.
If it was, the EULA for SQL server and oracle would have to include copyright permission for derivative works. That we do not see that should be a clear sign that the scope of copyright has not reached that far yet.
As for the "part of a web service"-bit, I'm not sure what the agpl's actual "reach" is. My understanding is that with a (modified) db under agpl powering eg a web app runing in php, the end users (accessing only the web server) would not be entitled to the db source. If the agpl covered the web srrver itself or a php library on the other hand, the users would be entitled to that code?
Similarly if one sold a modified db-as-a-service, modifications would be covered by the agpl.