> and why does it work with mySQL NULL = NULL but NULL + 1 = 0!

NULL + 1 will not result in 1 or in 0.

NULL + 1 = NULL

If this isn't the same in MySQL, then blame MySQL.

Do note, that "firebird-general" is not a real support list,
"firebird-support" is.

With regards,

Martijn Tonies
Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL
Server.
Upscene Productions
http://www.upscene.com