Rule 1: Each entity become a Table.
Rule 2: In the case entities are linked by 1:1 relation tables must have the
same key.
Rules 3: In the case entities are linked by 1:n relation each table must
have it's own key, but the key on the 0:n (or 1:n) side go to the other
table on the 0:1 (or 1:1) side and become a foreign key.
Rules 4: In the case entities are linked by n:m relations a new table os
created, this intermediary table is know as a jointing table and must have
as primary key a combination of others table foreign key.
