《数据库系统基本概念教学》ch.ppt

Reduction to Relational Schemas Extended ER Features * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Figure 7.22 Figure 7.23 Figure 7.24 Figure 7.25 Figure 7.26 Figure 7.27 Figure 7.28 Figure 7.29 * * * * * * * * * * * * * * * * * * * * * * * * Schemas Corresponding to Aggregation (Cont.) For example, to represent aggregation manages between relationship works_on and entity set manager, create a schema eval_for (s_ID, project_id, i_ID, evaluation_id) Schema proj_guide is redundant provided we are willing to store null values for attribute manager_name in relation on schema manages E-R Design Decisions The use of an attribute or entity set to represent an object. Whether a real-world concept is best expressed by an entity set or a relationship set. The use of a ternary relationship versus a pair of binary relationships. The use of a strong or weak entity set. The use of specialization/generalization – contributes to modularity in the design. The use of aggregation – can treat the aggregate entity set as a single unit without concern for the details of its internal structure. How about doing another ER design interactively on the board? Summary of Symbols Used in E-R Notation Symbols Used in E-R Notation (Cont.) Alternative ER Notations Chen, IDE1FX, … Alternative ER Notations Chen IDE1FX (Crows feet notation) UML UML: Unified Modeling Language UML has many components to graphically model different aspects of an entire software system UML Class Diagrams correspond to E-R Diagram, but several differences. ER vs. UML Class Diagrams *Note reversal of position in cardinality constraint depiction ER vs. UML Class Diagrams ER Diagram Notation Equivalent in UML *Generalization can use merged or separate arrows independent of disjoint/overlapping UML Class Diagrams (Cont.) Binary relationship sets are repre