By Martin Gogolla
This article offers 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 so as to limit database states to constant ones. The textual content explains an implementation of the procedure selected within the good judgment 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 procedure is that it turns into attainable to end up language houses on a legitimate mathematical foundation.
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Additional resources for An Extended Entity-Relationship Model: Fundamentals and Pragmatics (Lecture Notes in Computer Science)
Therefore, the expressions -[ str I (str: BTS -[ tname(t) [ (t:TOWN) ]- t2 SWS -[ city(a) [ (a:LTS(addr(p)));(p:PEaSON) ]-) ]and -[ str [ (str: BTS -[ tname(t) [ (t:TOWN) ]- U BTS -[ city(a) I (a:LWS(addr(p))) ]- ; (p:PERSON)) ]- SECTION 3D. R A N G E S A N D D E C L A R A T I O N S 51 will generally evaluate differently with respect to duplicates. 5. (f:government(c)) is not a correct declaration since government(c) is not a set-valued term and consequently not a correct range. The intended formulation is the following declaration.
5. cname(c) is a term of sort string since c is a term of sort C O U N T R Y and cname a function cname : C O U N T R Y --+ string. This term evaluates to the name of the country assigned to the variable c by a given a. distance(la) is a term of sort real and evaluates for a given a to the distance of the town l a . T O W N from the river l a . 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.
We see that it is possible to have bags as target terms allowing nested bag-valued terms. 8 Remark: Type construction and inheritance There is a close connection between the notions of type construction and inheritance. 2. 2: Inheritance of input type attributes to output types and vice versa. If we have a term tin of entity type sin we can also build the term so, t(tin) of entity type so,,t and (assuming ao~t is an attribute of entity type s~,t) the term a~,t(s~,t(tin)) is well-formed. It is possible to apply attribute a~,t to a term of sort sin and, in some measure, this means entity type sin inherits attribute a~t via the type construction.