Question

Exercise 4.3.2 We may think of relationships in the E/R model as having keys, just as entity sets do. Let R be a relationship among the entity sets E1, E2, …,En. Then a key for R is a set K of attributes chosen from the attributes of E1, E2,…, En such that if (e1,e2,…,en) and (f1,f2,…,fn) are two different tuples in the relationship set for R, then it is not possible that these tuples agree in all the attributes of K. Now, suppose n=2; that is, R is a binary relationship. Also, for each I, let Ki be a set of attributes that is a key for entity set Ei. In terms of E1 and E2, give a smallest possible key for R under the assumption that:

R is many-many

R is many-one from E1 to E2.

R is many-one from E2 to E1.

R is one-one.

Answer 1

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