Primary keys as null

I made a mistake when replying to a message regarding whether a primary key column could be null. The correct answer is that this cannot be done in Informix Online (and later versions of SE), if the column is defined as a primary key (and not just as unique). Earlier versions of Informix did not support the primary key designation, and thus the only way to define a primary key was with a unique index, which would allow a null value (unless of course it was defined as not null as well). Tim