CGS2545 Chapter 5

Terms for Chapter 5

Relation : A named two-dimensional table of data.

Primary key : An attribute (or combination of attributes) that uniquely identifies each row in a relation.

Composite key : A primary key that consists of more than one attribute.

Foreign key : An attribute in a relation of a database that serves as the primary key of another relation in the same database.

Null : A value that may be assigned to an attribute when no other value applies or when the applicable value is unknown.

Entity integrity rule : No primary key attribute (or component of a primary key attribute) can be null.

Referential integrity constraint : A rule that states that either each foreign key value must match a primary key value in another relation or the foreign key value must be null.

Well-structured relation : A relation that contains minimal redundancy and allows users to insert, modify, and delete the rows in a table without errors or inconsistencies.

Anomaly : An error or inconsistency that may result when a user attempts to update a table that contains redundant data. The three types of anomalies are insertion, deletion, and modification.

Recursive foreign : key A foreign key in a relation that references the primary key values of that same relation.

Normalization : The process of decomposing relations with anomalies to produce smaller, well-structured relations.

Normal form : A state of a relation that results from applying simple rules regarding functional dependencies (or relationships between attributes) to that relation.

Functional dependency : A constraint between two attributes or two sets of attributes.

Determinant : The attribute on the left-hand side of the arrow in a functional dependency.

Candidate key : An attribute, or combination of attributes, that uniquely identifies a row in a relation.

First normal form : A relation that contains no multivalued attributes.

Second normal form : A relation in first normal form in which every nonkey attribute is fully functionally dependent on the primary key.

Partial functional dependency : A functional dependency in which one or more nonkey attributes are functionally dependent on part (but not all) of the primary key.

Third normal form : A relation that is in second normal form and has no transitive dependencies present.

Transitive dependency : A functional dependency between two (or more) nonkey attributes.

Synonyms : Two (or more) attributes having different names but the same meaning, as when they describe the same characteristics of an entity.

Alias : An alternative name used for an attribute.

Homonym : An attribute that may have more than one meaning.

Enterprise key : A primary key whose value is unique across all relations.

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Questions from Companion Website

Multiple Choice

1. In ___ normal form, any multivalued attributes have been removed.
Correct answer: first
All multivalued (or repeating groups) have been removed so there is a single value at the intersection of each row and column in a table.

2. In ____ normal form, any transitive dependencies have been removed.
Correct answer: third
All functional dependencies between nonkey attributes have been removed.

3. Another name for a synonym is a(n)
Correct answer: alias.
Another name for a synonym is an alias, which is an alternative name used for an attribute.

4. A(n) ____ is an attribute that uniquely identifies each row in a relation.
Correct answer: primary key
Underlining the attribute name designates a primary key.

5. A(n) ____ is an attribute in a relation that serves as a primary key of another relation in the same database.
Correct answer: foreign key
Foreign keys represent the relationship between two tables or relations.

6. Two common ways of expressing a conceptual schema are
Correct answer: graphical and text (narrative).
Text is simpler; however graphical better expresses constraints.

7. The ____ guarantees that every primary key attribute is not null.
Correct answer: entity integrity rule
A null is an absence of a value.

8. The ____ states that a foreign key must either match a primary key value in another relation or it must be null.
Correct answer: referential integrity constraint
Referential integrity maintains consistency among the rows of two relations.

9. A(n) ____ contains minimal data redundancy.
Correct answer: well-structured relation
When redundancy is reduced, inconsistencies or anomalies are reduced.

10. A(n) ____ is a constraint between two attributes.
Correct answer: functional dependency
Functional dependency between two attributes is usually represented by an arrow.

11. This is similar to an object-oriented database’s object identifier.
Correct answer: enterprise key
An enterprise key is similar to an object identifier; its value is unique across all relationships in a database.

12. Which is NOT a problem that may arise in view integration or merging?
Correct answer: functional dependency
Synonyms, homonyms, transitive dependencies, and hidden supertype/subtype relationships may be revealed during view integration.

13. A(n) ____ corresponds to a column in the relation.
Correct answer: attribute
Each column corresponds to an attribute; each row corresponds to a record.

14. ____ entities are entities that cannot exist except with an identifying relationship with a regular entity type.
Correct answer: Weak
Weak entities are identified by a rectangle with a double line.

15. ____ entities are also called gerunds.
Correct answer: Associative
Associative entities (gerunds) are formed from many-to-many relationships between other entity types.

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True/False Questions

1. The hierarchical data model is the most commonly used model in contemporary database applications.
Correct answer: FALSE
The relational data model is the most commonly used model.

2. The relational model was first introduced in the 1980s by E.F. Codd.
Correct answer: FALSE
The relational model was introduced in the 1970s.

3. Another name for a relation is a table.
Correct answer: TRUE
A table or relation consists of named columns and rows.

4. EMPLOYEE1(Emp_ID, Name, Dept, Salary) describes four tables.
Correct answer: FALSE
EMPLOYEE1(Emp_ID, Name, Dept, Salary) describes one table with four columns.

5. A composite key is an attribute in one table that acts as a primary key in another table.
Correct answer: FALSE
A composite key is a primary key that consists of more than one attribute.

6. The entity integrity rule states that no primary key attribute (or component) may be null.
Correct answer: TRUE
This ensures that every relation has a unique primary key.

7. Redundancies in a table may cause anomalies
Correct answer: TRUE
Redundancies may result in errors or inconsistencies (anomalies).

8. A weak entity type can exist independent of its relationship with other entity types.
Correct answer: FALSE Weak entity types depend on another entity type (owner) to exist.

9. The procedure for representing relationships depends solely on the cardinalities of the relationships.
Correct answer: FALSE The procedure depends on the cardinalities of the relationships and the degree of the relationships

10. In 1:M relationships, the entity on the one-side of the relationship becomes a foreign key in the table of the many-side of the relationship.
Correct answer: TRUE
A mnemonic to remember this is: the primary key MIGRATES to the MANY side.

11. A recursive foreign key is used in a unary relationship.
Correct answer: TRUE
A recursive foreign key is a foreign key in a relation that references the primary key values of that same relation.

12. Normalization is the process of decomposing relations to produce smaller, well-structured relations.
Correct answer: TRUE
Normalization is the process for deciding which attributes should be grouped together in a relation.

13. A determinant is a constraint between two attributes or sets of attributes.
Correct answer: FALSE
Functional dependency is the constraint between attributes.

14. A determinant is an attribute or combination of attributes that uniquely identifies a row in a relation.
Correct answer: FALSE
A candidate key uniquely identifies a row in a relationship.

15. An attribute having more than one meaning is called a synonym.
Correct answer: FALSE
An attribute that has more than one meaning is a homonym.

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Essay Questions

1. What are the three components of the relational data model?
The relational data model components are a data structure, data manipulation, and data integrity. Data are organized in tables, with rows and columns. Powerful operations (using SQL) are used to manipulate data. Facilities are included to specify rules that maintain the integrity of the data.

2. What properties make a table a relation?
Relations have several properties that distinguish them from nonrelational tables. Each table in a database has a unique name and there can be no multivalued attributes in a table. Each row is unique; each attribute within a table has a unique name. The sequence of the columns or the rows is insignificant.

3. What is normalization and why is it done?
Normalization is the process for deciding which attributes should be grouped together in a relation. It is done to validate and improve logical design so that duplication of data and related problems can be avoided.

4. Why should you understand how to merge relations?
There are three reasons why merging is important.
On large projects, the work of several teams may come together during a logical design and merging will need to be done.
Integrating existing databases with new information requirements often leads to merging.
New data requirements may arise as the business changes, so there will be a need to merge new relations with existing relations.

5. What are the three major steps in the logical database design process?
The three major steps in the logical database design process are to
transform E-R or ERR diagrams to relations,
to normalize the relations,
and to merge the relations.


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