tailieunhanh - PATTERNS OF DATA MODELING- P50

PATTERNS OF DATA MODELING- P50: Models provide the means for building quality software in a predictable manner. Models let developers think deeply about software and cope with large size and complexity. Developers can think abstractly before becoming enmeshed in the details of writing code. Although models are beneficial, they can be difficult to construct. That is where patterns come in. Patterns provide building blocks that help developers construct models faster and better. | Explanation of the IDEF1X Notation 233 Generalization Generalization is the organization of entity types by their similarities and differences. The supertype holds common attributes and relationships. The subtypes add specific attributes and relationships. Each subtype inherits the attributes and relationships of its supertype. The IDEF1X notation for generalization is a circle with a double line underneath. The double line indicates that the generalization is exhaustive each occurrence of a supertype must correspond to a subtype. IDEF1X also supports a single horizontal line indicating that a generalization is not exhaustive. As a matter of style all the generalizations in this book are exhaustive. The attribute next to the circle is called a discriminator and indicates the subtype that elaborates each supertype record. Figure shows two generalizations. One generalization has a supertype of Actor and subtypes of TangibleActor ActorRole and ActorRoleType. The other generalization has a supertype of TangibleActor and subtypes of Person Application and Organization. Note that as a matter of style we rename propagated keys for generalization so that the primary key name corresponds to the entity type name. Such a practice makes it easier to understand foreign keys for a large model where there are various relationships to different levels of a generalization. Actor Person Application Organization personID FK f applications FK y organizations FK Figure IDEF1X generalization. 234 Appendix B Explanation of the IDEF1X Notation Bibliographic Notes The explanation of IDEF1X in this appendix is abridged and only covers the constructs used in this book. Bruce-1992 has a more extensive discussion of IDEF1X. Blaha-2001 compares IDEF1X to other notations. References Blaha-2001 Michael Blaha. A Manager s Guide to Database Technology. Upper Saddle River New Jersey Prentice-Hall 2001. Bruce-1992 Thomas A. Bruce. Designing Quality Databases with IDEF1X Information Models.