NOT NULL Issue

Hi

Why is ''  not considered null is postgres (8.1.3) Currently I have to
use the following workaround where I have zero length strings in char
fields.

select * from  security.users  where length(us_username)=0;
Surely this a null. Apparently not in Postgres.

Currently I have to use the following SQL to pick up zero length strings:
alter table security.users add constraint notnull_username
check(us_username <> '');

Thanks
Gustav