tailieunhanh - Database Design Using Entity-Relationship Diagrams phần 7

Tất cả các mối quan hệ mà chúng tôi đã xử lý vậy, đến nay trong các chương trước đó đã được các mối quan hệ nhị phân. Mặc dù mối quan hệ nhị phân có vẻ tự nhiên cho hầu hết chúng ta, trong thực tế đôi khi nó là cần thiết để kết nối ba hoặc nhiều hơn các thực thể. Nếu một mối quan hệ kết nối ba thực thể, nó được gọi là ba yếu tố "3-phân." | Chapter 7 Ternary and Higher-Order ER Diagrams Overview All relationships that we have dealt with thus far in previous chapters have been binary relationships. Although binary relationships seem natural to most of us in reality it is sometimes necessary to connect three or more entities. If a relationship connects three entities it is called ternary or 3-ary. If a relationship connects three or more entities n entities it is called an nary relationship where n equals the number of entities that participate in the relationship. n-ary relationships are also referred to as higher-order relationships. In this chapter we consider relationships that connect three or more entities. First we look at ternary 3-ary relationships. Ternary relationships arise for three main reasons 1 if we have intersection attributes that require three different entities to identify the attribute 2 if we have a relationship of a relationship and 3 by reverse-engineering. Because we discuss reverseengineering in Chapter 9 we will not discuss the development of ternary relationships from reverse-engineering in this chapter. In this chapter we first discuss how intersection attributes create ternary relationships and then look at the structural constraints of ternary relationships. Next we discuss how ternary and other n-ary relationships do not preclude binary relationships and how some ternary diagrams can be resolved into binary relationships. The development of ternary relationships from relationships of relationships is also discussed. Step 6 of the ER design methodology is also redefined in this chapter to include ternary and other higher-order relationships. Binary or Ternary Relationship Ternary relationships are required when binary relationships are not sufficient to accurately describe the semantics of an association among three entities. In this section we explain the difference between a binary and a ternary relationship with the help of an example and also show how an intersection