03datamodel-ER课件.pptVIP

Data Models Categories of data models: 1.Conceptual (high-level, semantic) data models: Provide concepts that are close to the way many users perceive data. Conceptual data models use concepts such as entities, attributes, and relationships. 2.Physical (low-level, internal) data models: Provide concepts that describe details of how data is stored in the computer by representing information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records efficient. 3.Implementation (representational) data models: Provide concepts that fall between the above two, balancing user views with some computer storage details. The models used here are most frequently in traditional commercial DBMSs, and they include the widely-used relational data model, network and hierarchical ,Object-based data models (Object-oriented and Object-relational) A simplified diagram to illustrate the main phases of database design. Data Model should : Simulate the real world comparatively truly Be easy to understand Be easy to be implemented by computer Abstraction and Modeling Two steps of the abstraction and modeling: The objective objects in the real world is abstracted to a concept model. Concept model is converted to a data model that supported by some DBMS. The concept model is a middle layer between the real world and the machine world. E-R Diagram With Composite, Multivalued, and Derived Attributes Two entity types, EMPLOYEE and COMPANY, and some member entities of each. Initial conceptual design of the company database Roles Entity sets of a relationship need not be distinct The labels “manager” and “worker” are called roles; they specify how employee entities interact via the works_for relationship set. Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship Some relationship in