You have a hell of a lot of alternatives here.
A) mircscripts.org -> mysql socket script. It works via signals but is slow slow slow!
B) mysql.dll on mircscripts.org -> Works fine; but not with mysql 4.1.0 or greater I've found
C) A COM Object -> there are a few out there, but many are under development so are buggy; or return a resultset as an array of result objects; which mirc can't deal with.
You can work via vbscript or whatever; and that has the advantage of running seperately from mirc (so it doesn't freeze up).
D) myODBC + COM object -> Its slow but allows you to read/write from/to any odbc database.
Find the com objects with google, or mysql.com