On Tuesday 27 May 2008, Larry Garfield wrote:
Drupal currently cannot connect to more than one database type at a time; All databases must be either MySQL or PostgreSQL. While one could write an MS SQL driver for Drupal's current database implementation, there's no guarantee that it would work with the vast array of contrib modules out there; in fact, odds are against you. MS SQL, because AFAIK it doesn't require any additional field handling, is one of the few that are even possible to think about. You would have to move the entire Drupal system over to MS SQL, though.
I forgot to mention, that limitation applies to Drupal's own database layer only. You can connect to MS SQL or anything else you want as long as you are not using Drupal's database layer for it; that could be a problem or not depending on your use case; (The node table is pretty much bound to only being useful via Drupal's database layer, but if you're doing extensive programming of your own you can point those database calls wherever. I recommend PDO as the tool to use for any extra-Drupal database access.)