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Database Design Using Entity-Relationship Diagrams phần 4

Ghi một hàng cho mỗi giá trị của thuộc tính multivalued cùng với chìa khóa từ bảng ban đầu. Loại bỏ các thuộc tính đa giá trị từ bảng ban đầu. Sử dụng quy tắc này lập bản đồ, dữ liệu trên sẽ được ánh xạ tới hai mối quan hệ: một mối quan hệ với các thuộc tính đa giá trị và quan hệ với các thuộc tính đa giá trị cắt bỏ. | English Descriptions We would now like to tighten the grammar that describes how a relationship affects entities using our structural constraints and to adopt a standard way of stating the relationship. The standard language should appear on the model or at least with it. Further using a standard language approach to describe the ER diagrams allows us to not only close the loop with the user in the systems analysis process but also facilitates feedback and nails down the exact meaning of the relationship. In the Chen-like model the double lines define full participation as in automobiles fully participate in the drive relationship. Better yet the double lines invite us to state the relationship as Automobiles must be driven by one and only one student. The must part comes from the full mandatory participation and the one part from the cardinality. The grammar for describing partial or optional relationship for the STUDENT entity to the AUTOMOBILE entity would be Students may drive one and only one automobile. The may comes from the single line leaving the STUDENT entity box and the one and only one part comes from the cardinality. The point is that when expressing the sense of the ER diagrams one uses the language that conveys what the relationship really means i.e. students may drive one automobile and automobiles must be driven by one and only one student . A graphic on how to read an ER diagram is presented in Figure 4.3. Figure 4.3 An ER Diagram of the STUDENT-AUTOMOBILE Database. Translating the Diagram into English Tighter English We strongly recommend that an English sentence accompany each diagram to reinforce the meaning of the figure. Refer to Figure 4.3. English is often an ambiguous language. The statement that Automobiles must be driven by one and only one student. actually means that Automobiles which are in the database must be driven by one and only one student. It does not mean that One particular student drives some automobiles. Another way to put