Chapter 6: Entity-Relationship Model

Question 1

What drives everything done in Databases?

Answer 1

The business

Question 2

Who was the Entity-Relationship Model designed for?

Answer 2

The general public, but it is still as powerful as the relational and RDB models.

Question 3

What is the purpose of the ER model?

Answer 3

It is supposed to allow a variety of experts to contribute regardless of their technical knowledge about databases.

Question 4

How can a database be modeled?

Answer 4

A database can be modeled as a collection of entities and a relationship among entities.

Question 5

What is an entity?

Answer 5

An entity is an object that exists and is distinguishable (avoiding duplicates) from other objects.

Question 6

What is an entity set?

Answer 6

A set of entities of the same type that share the same properties.

Question 7

What are attributes?

Answer 7

Essential characteristics that are determined by the business. Entities have attributes.

Question 8

What is a relationship?

Answer 8

An association among several entities. It is modeled by Keys in a relationship set.

Question 9

What is a relationship set?

Answer 9

A mathematical relation among at least 2 entities, each taken from entity sets.

Question 10

What is an attribute?

Answer 10

An attribute is a possible property of a relationship set. Entities are represented by sets of attributes.

Question 11

What is the degree of a relationship set?

Answer 11

It is the number of entity sets that participate in a relationship set. 2 sets is binary, etc. Relationship sets that are more than binary are rare.

Question 12

What is the domain of an attribute?

Answer 12

The domain is the set of permitted values (legal values) for each attribute.

Question 13

How are multiple values handled in RDB?

Answer 13

They are handled by linking tables together, breaking down complex attributes into its atomic subcomponents.

Question 14

What type of attributes are there?

Answer 14

Atomic (simple) and complex.

Question 15

What type of attribute must each attribute in an entity set be?

Answer 15

Simple (atomic)

-in RDB this looks like each row in the table being a simple attribute

Question 16

Who determines if an attribute is simple of composite?

Answer 16

THE BUSINESS

Question 17

What is the purpose of mapping cardinality constraints?

Answer 17

To express the number of entities to which another entity can be associated via a relationship set.

Question 18

For a binary relationship what are the types of cardinality mapping that can be done?

Answer 18

• one to one
• one to many
• many to one
• many to many

Question 19

What is one to one cardinality?

Answer 19

An element in A is mapped directly to a single attribute in B

Question 20

What is one to many cardinality?

Answer 20

An element in A is mapped to multiple elements in B

Question 21

What is many to one cardinality?

Answer 21

Many elements in A is mapped to one element in B

Question 22

What is many to many cardinality?

Answer 22

When many elements in A are mapped to many elements in B

Question 23

Name the 3 types of keys that exist.

Answer 23

• super key: a set of one or more attributes whose values uniquely determine each entity
• candidate key: the minimal super key
• primary key: a candidate key that is selected to be the main identifier

Question 24

What is the purpose of keys?

Answer 24

To avoid duplicates and maintain uniqueness within the set.

Question 25

What are the characteristics of keys?

Answer 25

1. unique
2. short
3. permanent

Question 26

What is a key?

Answer 26

a set of attribute(s), they are a subset of an attribute