An E-R scheme may de ne 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:
  1. 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)
  2. 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)
  3. 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)
  4. 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.