Up: The Entity-Relationship Model
An E-R scheme may define certain constraints to which the contents of a
database must conform.
- Mapping Cardinalities:
express the number of entities to which another entity can be associated
via a relationship.
For binary relationship sets between entity sets A and B, the mapping
cardinality must be one of:
The appropriate mapping cardinality for a particular relationship set
depends on the real world being modeled.
(Think about the CustAcct relationship...)
An entity in A is associated with at most one entity in B, and an entity
in B is associated with at most one entity in A. (Figure 2.3)
An entity in A is associated with any number in B.
An entity in B is associated with at most one entity in A. (Figure 2.4)
An entity in A is associated with at most one entity in B.
An entity in B is associated with any number in A. (Figure 2.5)
Entities in A and B are associated with any number from each other.
- Existence Dependencies:
if the existence of entity X depends on the existence of entity Y, then
X is said to be existence dependent on Y.
(Or we say that Y is the dominant entity and X is the subordinate
- Consider account and transaction entity sets,
and a relationship log between them.
- This is one-to-many from account to transaction.
- If an account entity is deleted, its associated transaction
entities must also be deleted.
- Thus account is dominant and transaction is subordinate.
Page created and maintained by Osmar R. Zaï ane
Sun Sep 10 17:02:35 PDT 1995