DB design using designer/2000???

Hi,

This is a database design question for all the database experts out there.

         The question is about the Designer/2000, when I use the  "Database design wizard" to convert the entity relationship diagram that I designed, it treat the entity relatioship "many to 1(O:O)" is same as "many to 1(M:M)", my question is if there is no intersection table between the entities of the many to 1(O:O) relationship, this means that there will be the null valuse exist in the foreign key field of the "many" side entity's mapping table, how does the "referential integrity" constraints be enforced in this case, and have the null value in the relation is against the principle of databse design. why the Oracle designs the
CASE tool in this way??? What is the concept behind the design.

        If I really need a intersection table between the many to 1(O:O) entities, is there a way I can create one after I perform the "database design wizard" to create the mapping table base my ER diagram.  

        Thanks in advantance
					Katherine