数据库系统基础教程第四章答案(12)

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4.2.1

Redundancy: The owner address is repeated in AccSets and Addresses entity sets.

Simplicity: AccSets does not serve any useful purpose and the design can be more simply represented by creating many-to-many relationship between Customers and Accounts.

Right kind of element: The entity set Addresses has a single attribute address. A customer cannot have more than one address.

Hence address should be an attribute of entity set Customers.

Faithfulness: Customers cannot be uniquely identified by their names. In real world Customers would have a unique attribute such as ssNo or customerNo

4.2.2

Studios and Presidents can be combined into one entity set Studios with Presidents becoming an attribute of Studios under following circumstances:

1. The Presidents entity set only contains a simple attribute viz. presidentName. Additional attributes specific to Presidents might justify making Presidents into an entity set.

4.2.3

4.2.4 The entity sets should have single attribute.

a) Stars: starName

b) Movies: movieName

c) Studios: studioName. However there exists a many-to-many relationship between Studios and Contracts. Hence, in addition, we need more information about studios involved. If a contract always involves two studios, two attributes such as producingStudio and starStudio can replace the

Studios entity set. If a contact can be associated with at most five studios, it may be possible to replace the Studios entity set by five attributes viz. studio1, studio2, studio3, studio4, and studio5. Alternately, a composite attribute containing concatenation of all studio names in a contact can be considered. A separator character such as "$" can be used. SQL allows searching of such an attribute using query like '%keyword%'

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