Web-DSS-Chapter-03

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44 CHAPTER 3 ■ Entity-Relationship Modeling 3.8 Unary Relationship As we mentioned in Section 3.5, a unary relationship (R ∈ E 1 × E 1 ) is an association between two entities of the same entity type. Figure 3.12(a) displays the E-R diagram of a unary relationship IsMarriedTo. Whenever two people in the entity type PERSON get married, the relationship instance IsMarriedTo is created. The Date of marriage is an attribute of this relationship. Since a person can only be married to one other person, marriage is a one-to-one relationship. Furthermore, since a person can be unmarried, the minimum cardinality of the IsMarriedTo relationship is zero. Figure 3.12(b) depicts several relationship instances of this relationship type. Each relationship instance (r1, r2, r3, and r4) connects two instances in PERSON. The lines allow us to PERSON IsMarriedTo Date (a) PERSON p1 p2 p3 p4 p5 p6 p7 p8 p9 IsMarriedTo r1 r2 r3 r4 p10 p11 p12 (b) Figure 3.12 The unary one-to-one relationship.
SECTION 3.8 ■ Unary Relationship 45 read relationships between entity instances. For example, r1 suggests that person p1 is married to person p5, and so forth. Figure 3.13(a) illustrates another example of a unary relationship, expressed by the relationship instance Supervises. This relationship instance exists whenever an employee supervises another employee. The relationship Supervises is a one-to-many relationship since an employer can supervise many employees but a supervisee can have only one supervisor. The minimum cardinality for supervising is zero (an employee may not supervise anyone) but the minimum cardinality of being supervised is one (every employee must be supervised). Figure 3.13(b) shows several relationship instances of this relationship type. Each relationship instance (r1, r2, r3, and r4) connects two entity instances in EMPLOYEE; these are the supervisor instance and the supervisee instance. For example, r2 suggests that employee e3 supervises employee e7, and so forth. Employee Supervises (a) e1 e2 e3 e4 e5 e6 e7 e8 e9 r1 r2 r3 r4 e10 e11 e12 (b) Figure 3.13 The unary one-to-many relationship.
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: SECTION 3.7 ■ Cardinality of a Re
Page 15 and 16: SECTION 3.9 ■ Binary 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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