As far as I know, when I build a desktop/server app using any GPL code like MySQL I will have to release the source code of my software under the GPL.
If I want to develop a web-app using MySQL, my code will link against the MySQL libraries. Must I release the sourcecode of my webapp in this situation to be in accordance with the GPL?
Provided that you keep it server-only(and therefore "private"), you don't have to release it as GPL. But as soon as it reaches public desktops, it can no longer be considered an internal build, and therefore source code is needed.
So you can: