Exercise 2. In this and following exercises we shall consider two design options in the E/R model for describing births. At a birth, there is one...
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E! Exercise 2.2.5: In this and following exercises we shall consider two design
options in the E/R model for describing births. At a birth, there is one baby
(twins would be represented by two births), one mother, any number of nurses,
and any number of doctors. Suppose, therefore, that we have entity sets Babies,
Mothers, Nurses, and Doctors. Suppose we also use arelationship Births, which
connects these four entity sets, as suggested in Fig. 2.15. Note that a tuple of
the relationship set for Births has the form (baby, mother, nurse, doctor) If there is more than one nurse and/or doctor attending a birth, then there will
be several tuples with the same baby and mother, one for each combination of
nurse and doctor. 46 CHAPTER 2. THE ENTITYeRELA'HONSHIP DATA MODEL Figure 2.15: Representing births by a multiw ay relationship There are certain assumptions that we might wish to incorporate into our
design. For each, tell how to add arrows or other elements to the E/R diagram
in order to express the assumption. a) For every baby, there is a unique mother. b) For every combination of a baby, nurse, and doctor, there is a unique
mother. c) For every combination of a baby and a mother there is a unique doctor. Figure 2.16: Representing births by an entity set ! Exercise 2.2.6: Another approach to the problem of Exercise 2.2.5 is to con-
nect the four entity sets Bsbaes, Mothers, Nurses, and Doctors by an entity set
Butts, with four relationships, one between Births and each of the other entity
sets, as suggested in Fig. 2.16. Use arrows (indicating that certain of these relationships are many-one) to represent the following conditions: a) Every baby is the result of a unique birth, and every birth is of a unique
baby. b) In addition to (a), every baby has aunique mother.

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