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:
- One-to-one: 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.
- One-to-many: An entity in A is associated with any number in B. An entity in B is associated with at most one entity in A.
- Many-to-one: An entity in A is associated with at most one entity in B. An entity in B is associated with any number in A.
- Many-to-many: Entities in A and B are associated with any number from each other.
The appropriate mapping cardinality for a particular relationship set depends on the real world being modeled. (Think about the CustAcct relationship…)
- 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 thesubordinate entity.)For example,
- 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.