By Martin Gogolla
This textual content provides a accomplished advent 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 knowledge version the textual content introduces a language for the formula of constraints that allows you to limit database states to constant ones. The textual content explains an implementation of the procedure selected within the common sense programing language PROLOG and discusses during this context the computational energy 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
1 are both terms of sort T O W N having the single free variable la. Both terms evaluate to the town that participates in the relationship assigned to the variable la by a given a. S E C T I O N 3C. T E R M S A N D F O R M U L A S 43 3. WATERS(r) is a term of sort W A T E R S having the single free variable r. This term evaluates for a given a to the water corresponding to the river assigned to r by a. Indeed, we have assumed every river to be also a water (by an additional constraint). The above is not a correct term in the calculus of [Hohg0] because it does not allow this form of conversion.
The function sort determines the sort of every term. The function free yields the set of free variables for a given term. The free variables are in fact the ones that 42 C H A P T E R 3. E X T E N D E D E N T I T Y - R E L A T I O N S H I P CALCULUS have to be given a value by an assignment. The assignments to non-free variables are not essential for the evaluation of terms. In the cases (i) - (v) the free variables are obvious. , dk. For the sake of simplicity we assume a formula f being omitted to be the tautology true.
R I V E R for a concrete relationship assigned to the variable la by a. The variable c is free in cname(c) and the variable la is free in distance(la). 6. pname(head(c)) is a term of sort string, since we have head : C O U N T R Y - + - P E R S O N and pname : P E R S O N -+ string, c is the only free variable. This term evaluates to the name of the head of the country assigned to c by a given a. 7. ministers(c) is a term of sort list(PERSON) having the single free variable C. 8. CNT(ministers(c)) is a term of sort int having the single free variable c.