ER and EER to Relational Mapping

• Step 1: Mapping of Regular Entity Types - each strong entity becomes a relation (table) in the relational schema, one of the candidate keys is chosen for the primary key
• Step 2: Mapping of Weak Entities - each weak entity becomes a relation (table) in the relational schema, the primary key from the identifying entity is concatenated in to the weak entity relation, becoming a foreign key in the weak relation.  The primary key of the weak relation is the weak key together with the foreign key.  I.e., there is a composite key for this relation.
• Step 3: Mapping of Binary 1:1 Relationship Types - there are 3 possible approaches here: 1) Foreign key approach 2) Merged relation approach 3)Cross-reference or relationship relation approach.  If the Foreign Key approach is used, any attributes on the 1:1 relationship will also be mapped over and become attributes in the same relation in which the primary key was brought into as a foreign key.
• Step 4: Mapping of Binary 1:N Relationship Types - The primary key from the 1 side of the relationship is concatenated into the relation on the many side of the relationship, becoming a foreign key in that relation.  Also, any attributes on the 1:N relationship will also be mapped over and become attributes in the entity on the many (N) side of the relationship.  I.e., the attributes will become attributes in the same relation in which the primary key was brought into as a foreign key.
• Step 5: Mapping of Binary M:N Relationship Types - A new relation is created ( aka an "association relation or association object").  The primary key of this new relation is the concatenated keys from both sides of the M:N relation.  I.e., there is a composite key to this new relation.  Also, any attributes on the M:N relationship will become attributes in the new association relation.
• Step 6: Mapping of Multivalued Attributes - Each multivalued attribute gets a new relation  R and the primary key from the entity (E) for which this was a multivalued attribute is concatenated into this new relation as a foreign key.  The key of this new relation is the primary key from E together with the multivalued attribute now in R.
• Step 7: Mapping of N-ary Relationship Types - similar to M:N n- we get a new relation, all primary keys from the related entities are concatenated in to the new relation to form its primary key.
• Step 8: Options for Mapping Specialization or Generalization.

  Option 8A: Multiple relations - superclass and subclasses

  Option 8B: Multiple relations - subclass relations only

  Option 8C: Single relation with one type attribute

  Option 8D: Single relation with multiple type attributes