Mapping Constraints

Posted By on December 12, 2014


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2- PHASE COMMIT PROTOCOL
Specialization

Mapping Constraints

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:
    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.
    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.
    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.
    4. 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.
2- PHASE COMMIT PROTOCOL
Specialization

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