tailieunhanh - PATTERNS OF DATA MODELING- P48

PATTERNS OF DATA MODELING- P48: 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. | Appendix A Explanation of the UML Notation The UML Unified Modeling Language is a graphical language for modeling software development artifacts. The UML encompasses about a dozen notations of which one the class model concerns data structure. The class model sets the scope of data and level of abstraction for subsequent development. The Object Management Group has been actively working towards standardizing all of the UML notations. The UML class model is an object-oriented notation. Nevertheless the class model is entirely suitable for databases and actually derives from the Chen notation Chen-1976 . The Chen notation and its derivatives have been influential in the database community but there are many dialects and a lack of consensus. This UML class model is just another Chen dialect but one that has the backing of a standard. The class model also has several helpful features such as ordering qualifiers aggregation composition to be discussed not found in most other Chen dialects. This appendix focuses on the class model it ignores the other UML notations as they are less relevant to database applications. I try to avoid object-oriented jargon and use entityrelationship terminology where possible. The major concepts in the class model are entity types relationships and generalizations. Entity Type An entity UML term is object is a concept abstraction or thing that can be individually identified and has meaning for an application. An entity type UML term is class describes a group of entities with the same attributes kinds of relationships and intent. Figure shows four entity types. The UML symbol for an entity type is a box with the name of the entity type in bold font toward the top. The second portion of the box the three entity types to the left of Figure shows attribute names. An attribute describes a value held by each entity of an entity type. Account has three attributes Actor has three attributes and PhysicalPart has one attribute.