Web-DSS-Chapter-03

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46 CHAPTER 3 ■ Entity-Relationship Modeling PART Uses Number (a) PART q1 q2 q3 q4 q5 q6 q7 q8 q9 q10 (b) Figure 3.14 The unary many-to-many relationship. Uses r1 r2 r3 r4 r5 Figure 3.14(a) provides an example of a unary relationship in which certain parts are used to make other parts. The attribute Number stores the number of parts used. The relationship Uses quantifies the number of parts used to make any other part. The Uses relationship is a many-to-many relationship since a part can use many parts and can be used in the making of many parts. The minimum cardinality of this relationship is zero because a part may not use any other part and may not be used by any other part. Figure 3.14(b) displays five instances of this relationship. Observe that several lines can emanate from a part instance and several lines terminate at a part instance. 3.9 Binary Relationship A binary relationship, or an association between two entity types, is the most common form of a relationship expressed by an E-R diagram. Recall that a binary relationship is R ∈ E 1 × E 2 in which E 1 and E 2 are two different entity types. The examples of binary relationships with the relationship instances are presented in Figure 3.15. Notice that each relationship instance obeys a
SECTION 3.9 ■ Binary Relationship 47 basic characteristic of the binary relationships; in other words, each relationship instance is connected to exactly two entity instances of different entity types. By applying what we have learned earlier in this chapter, we should be able to determine the cardinalities of these relationships quite easily. The relationship in Figure 3.15(a) is a one-to-one relationship since we assume that an employee can manage at most one department and each department is managed by at most one employee. The minimum cardinality can be determined EMPLOYEE Manages DEPARTMENT (a) EMPLOYEE Manages DEPARTMENT e1 e2 e3 e4 e5 e6 e7 e8 r1 r2 r3 r4 r5 d1 d2 d3 d4 d5 (b) DateOfOrder CUSTOMER Places ORDER (c) (continues) Figure 3.15 (a) and (b): Binary one-to-one relationships; (c) and (d): Binary one-to-many relationships; (e) and (f): Binary many-many relationships.
Page 1 and 2: three Entity-Relationship Modeling
Page 3 and 4: SECTION 3.3 ■ Entity 35 Sends SUP
Page 5 and 6: SECTION 3.4 ■ Attributes 37 E-R d
Page 7 and 8: SECTION 3.5 ■ Relationships 39 St
Page 9 and 10: SECTION 3.7 ■ Cardinality of a Re
Page 11 and 12: SECTION 3.7 ■ Cardinality of a Re
Page 13: SECTION 3.8 ■ Unary Relationship
Page 17 and 18: SECTION 3.10 ■ Ternary Relationsh
Page 19 and 20: SECTION 3110 ■ Attributes of Rela
Page 21 and 22: SECTION 3.12 ■ Associative Entiti
Page 23 and 24: SECTION 3.14 ■ Enhanced Entity-Re
Page 25 and 26: SECTION 3.15 ■ Superclass, Subcla
Page 27 and 28: SECTION 3.15 ■ Superclass, Subcla
Page 29 and 30: SECTION 3.16 ■ Generalization and
Page 31 and 32: SECTION 3.17 ■ Participation and
Page 33 and 34: SECTION 3.17 ■ Participation and
Page 35 and 36: SECTION 3.18 ■ Superclass./Subcla
Page 37 and 38: SECTION 3.21 ■ Summary 69 3.20 In
Page 39 and 40: SECTION 3.22 ■ Exercises 71 12. U
Page 41 and 42: SECTION 3.22 ■ Exercises 73 7.
Page 43 and 44: SECTION 3.22 ■ Exercises 75 curre
Page 45 and 46: SECTION 3.22 ■ Exercises 77 below
Page 47 and 48: SECTION 3.22 ■ Exercises 79 c. Fo
Page 49 and 50: SECTION 3.22 ■ Exercises 81 attri

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