By Carol Batini;Stefano Ceri;Shamkant B. Navathe
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Additional info for Conceptual Database Design: An Entity-Relationship Approach
The evolution of the database can be seen as the transition from one instance to another instance caused by some data modification operation. We use the term instance also for an element of the schema, to denote the collection of data that refers to that particular element. We understand from the context whether the term refers to the entire schema or to a single element. 14a represents three persons and four cars. Each car is owned by one person. 14a represents a particular state of the real world; however, it is possible to change that state.
If we assume that each day of the week can have any number of class meetings, then min-card (DAYMEETS) = 0 max-card (DAYMEETS) =n 3. If we assume that each classroom can have at most forty meetings per week (eight class meetings per day, five days per week), then min-card (CLASSROOM,MEETS) = 0 max-card (CLASSROOM,MEETS) = 40 As with binary aggregations, we completely characterize each participation of one class in an aggregation by indicating the two values of minimal and maximal cardinalities.
These models are less homogeneous than data models and not as easily comparable, because functional analysis is applied to very different problems, ranging from conventional data processing to scheduling and real-time control. In this book, we have emphasized data processing application, and from this context the dataflow model has emerged, becoming an industry standard. This model is simple and concise; furthermore, each element of the model is mapped to a distinct graphic symbol. More recently, the focus has moved from data models to design methodologies and tools; one may argue that a correct methodological approach is at least as important as the choice of data or function models.