By Martin Gogolla
This textual content 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 one can limit database states to constant ones. The textual content explains an implementation of the strategy 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 which means of the relational question language SQL. a pleasant characteristic of the method is that it turns into attainable to end up language houses on a valid mathematical basis.
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Additional resources for An Extended Entity-Relationship Model: Fundamentals and Pragmatics
At first, we do not only allow the form (v : r) but also a union (i) (v : rl U ... U rk). This form is necessary to express every term of the relational algebra in our calculus, and in particular the union of sets. , v , (in front of the colons) are declared (each (v~ : ri) may be a union of form (i)). , vn, plus the free variables of r , . Indeed, the free variables of rn are the free variables of the declaration, which must be disjoint from the variables declared in the declaration: free(d) M decl(d) = 0 holds for all declarations d.
U r , , a ) e #[DECL], iff a ( v ) 9 # [ R A N G E ] ( r l , a ) or ... or a ( v ) 9 #[RANGE](r,,a). ii. (v : rl U ... U r,; d,c~) 9 #[DECL], iff (c~(v) 9 #[RANGE](rl,c~) or ... or a ( v ) 9 # [ R A N G E ] ( r , , a ) ) and (d,~) 9 #[DECL]. At first, we do not only allow the form (v : r) but also a union (i) (v : rl U ... U rk). This form is necessary to express every term of the relational algebra in our calculus, and in particular the union of sets. , v , (in front of the colons) are declared (each (v~ : ri) may be a union of form (i)).
15: Type constructions as specializations. 15 coincide with town specializations to towns having a port and with town specializations to towns being capitals, respectively. This is an example of an entity 30 C H A P T E R 2. S T R U C T U R E OF D A T A A N D E N T I T I E S type which is the input for two type constructions. , an entity type which is the output of two type constructions is not allowed, because this would destroy the property that an entity has a uniquely determined construction path.