By Martin Gogolla
This textual content offers a finished creation to a longer Entity-Relationship version either on a conceptual and on a proper, mathematicallevel. as well as the primitives given by way of the information version the textual content introduces a language for the formula of constraints with a view to limit database states to constant ones. The textual content explains an implementation of the technique selected within the good judgment programing language PROLOG and discusses during this context the computational strength of the proposedcalculus. The prolonged Entity-Relationship calculus is used to outline the that means of the relational question language SQL. a pleasant characteristic of the strategy is that it turns into attainable to end up language houses on a legitimate mathematical basis.
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Additional info for An Extended Entity-Relationship Model: Fundamentals and Pragmatics
RIVER length : real RIVER, WATERS PERSON, TOWN SEA, LAKE, RIVER WATERS The elements of E N T T Y P E are called entity (or object) types, the elements of R E L T Y P E are the relationship types, and the elements of A T T R I B U T E are 24 C H A P T E R 2. S T R U C T U R E OF DATA A N D ENTITIES the attributes names, all known from the Entity-Relationship Model defined in [Che76]. , related entities. Therefore, we have the following situation in our current example: a. }, b. }, c. } d .... The semantics could look like this: a.
Yet another variant is offered by SDM in the form of set-operator defined subclasses, where the supertype is made up of subtypes by using the set operators union, intersection, or difference. We can model the case union o] subtypes with generalization. With respect to intersection or difference we have to employ explicit integrity constraints once again. On balance, it is our opinion that the concepts of aggregation, association, and specialization in their general form seem to be integrated. In contrast to other data models, we make a strict distinction between modeling primitives and structural restrictions.
In this sense, the type TEAM would be an association over PERSON since each team (instance) consists of a set of members (of type PERSON). 18. Thus, set-valued components completely support association, while other Entity-Relationship approaches generally do not go beyond simple setvalued attributes. On the other hand, some semantic data models extend the notion of association. For example, SDM provides expression-defined SECTION 219. 18: Representation of association. grouping , where the membership in one of the groups is defined by an expression, or enumerated grouping, where the members are explicitly itemized.
An Extended Entity-Relationship Model: Fundamentals and Pragmatics by Martin Gogolla