数据库系统基础教程第四章答案

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Solutions

Chapter 4 4.1.1

4.1.2

a)

b)

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c)

In c we assume that a phone and address can only belong to a single customer (1-m relationship represented by arrow into customer).

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d)

In d we assume that an address can only belong to one customer and a phone can exist at only one address.

If the multiplicity of above relationships were m-to-n, the entity set becomes weak and the key ssNo of customers will be needed as part of the composite key of the entity set.

In c&d, we convert attributes phones and addresses to entity sets. Since entity sets often become relations in relational design,

we must consider more efficient alternatives.

Instead of querying multiple tables where key values are duplicated, we can also modify attributes:

(i) Phones attribute can be converted into HomePhone, OfficePhone and CellPhone.

(ii) A multivalued attribute such as alias can be kept as an attribute where a single column can be used in relational design i.e. concatenate all values. SQL allows a query "like '%Junius%'" to search the multiple values in a column alias.

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4.1.3

4.1.4

a)

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b)

c)

The relationship "played" between Teams and Players is similar to relationship "plays" between Teams and Players.

4.1.5

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4.1.6 The information about children can be ascertained from motherOf and fatherOf relationships. Attribute ssNo is required since names are not unique.

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4.1.7

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4.1.8

a)

(b)

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4.1.9

Assumptions

A Professor only works in at most one department.

A course has at most one TA.

A course is only taught by one professor and offered by one department.

Students and professors have been assigned unique email ids.

A course is uniquely identified by the course no, section no, and semester (e.g. cs157-3 spring

09).

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4.1.10

Given that for each movie, a unique studio exists that produces the movie. Each star is contracted to at most one studio.

But stars could be unemployed at a given time. Thus the four-way relationship in fig 4.6 can be easily into converted equivalent relationships.

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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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4.2.5

From Augmentation rule of Functional Dependency,

given

B -> M (B=Baby, M=Mother)

then

BND -> M (N=Nurse, D=Doctor)

Hence we can just put an arrow entering mother.

a) Put an arrow entering entity set Mothers for the simplest solution (As in fig. 4.4, where a multi-way relationship was allowed, even though Movies alone could identify the Studio). However, we can display more accurate information with below figure.

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c)

Again from Augmentation rule of Functional Dependency,

given

BM -> D

then

BMN -> D

Thus we can just add an arrow entering Doctors to fig 4.15. Below figure represents more accurate information however.

4.2.6

a)

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b) Transitivity and Augmentation rules of Functional Dependency allow arrow entering Mothers from Births. However, a new relationship in below figure represents more accurate information.

c)

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