Mapping ER to relations

The rules to map an ER diagram to a set of relations (that is, the database schema) are almost straightforward, but not rigid. One could model an entity as an attribute, and then refine it to a relationship. An attribute that belongs to several entities can be promoted to be an independent entity. The most common rules are as follows (note that only basic rules have been covered, and the list is not exhaustive):

• Map regular entities to relations. If entities have composite attributes, then include all the subparts of the attributes. Pick one of the key attributes as a primary key.
• Map weak entities to relations. Include simple attributes and the subparts of the composite attributes. Add a foreign key to reference the identifying entity. The primary key is normally the combination of the partial key and the foreign key.
• If a relationship has an attribute and the relation cardinality is 1:1, then the relation attribute can be assigned to one of the participating entities.
• If a relationship has an attribute and the relation cardinality is 1:N, then the relation attribute can be assigned to the participating entity on the N side. 
• Map many-to-many relationships, also known as N:M, to a new relation. Add foreign keys to reference the participating entities. The primary key is the composition of foreign keys. 
• Map a multi-valued attribute to a relation. Add a foreign key to reference the entity that owns the multi-valued attribute. The primary key is the composition of the foreign key and the multi-valued attribute.