| Mapping Constraint |
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. (Figure
2.3)
- 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. (Figure 2.4)
- 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. (Figure 2.5)
- Many-to-many: Entities in A and B are associated with any number
from each other. (Figure 2.6)
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 the
subordinate 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.
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