In my LinksSQL, I added a field called "Price". It was defined as:
Price INT UNSIGNED DEFAULT NULL,

When a visitor added a link and left this field blank, there was no problem. When I viewed the link in admin, this field was blank.
However; if later he modified his link and left this field blank again, this field would be automaticly filled with '0'. I also couldn't modified it to be blank in admin.

Any idea? Thanks!