Week 2

Question 1

What is a relational data model?

Answer 1

A means of storing information about a database system in tables called relations

Question 2

What is a table in a relational data model?

Answer 2

A collection of tuples (rows) that share the same attributes

Question 3

What is a tuple in a relational data model?

Answer 3

A specific instance that relates to one ‘thing’ in the database system

Question 4

What is the domain of an attribute?

Answer 4

The set of allowed values that an object can take

Question 5

What is generalisation?

Answer 5

The process by which a lower level entity inherits all the attributes and relationship participation of the higher level entity it is linked to

Question 6

What is the benefit of generalisation?

Answer 6

Avoids the duplication of data

Question 7

What does the completeness constraint specify?

Answer 7

Whether an entity in a superclass entity set must belong to at least one of the subclass entity sets within a generalization

Question 8

What are the two ways an entity can participate in a superclass entity set?

Answer 8

Total, Partial

Question 9

What is total participation?

Answer 9

Where an etity must belong to one of the subclass entity sets

Question 10

What is partial participation?

Answer 10

Where an entity need not belong to one of the subclass entity sets

Question 11

What are inheritance constraints concerned with

Answer 11

Whether or not entities may belong to more than one subclass entity set within a generalization

Question 12

What is overlapping inheritance?

Answer 12

An entity can belong to more than one subclass entity set

Question 13

What is the inheritance called when an entity can belong to at most one subclass entity set?

Answer 13

Disjoint

Question 14

State ER design decisions we may have to make

Answer 14

• Whether it should be entity set or attribute
• Whether it should be entity set or relationship set
• The placement of attributes on entity sets or relationship sets
• The use of an n-ry relationship versus multiple binary relationship sets
• The use of a strong or weak entity set
• The use of generalisation

Question 15

What is a relationship set?

Answer 15

An action that occurs between entities

Question 16

Where should an attribute of many to many relationships be placed?

Answer 16

On the relationship set

Question 17

Where should an attribute of one to many relationship be placed?

Answer 17

On the relationship set or on the many-side entity set

Question 18

Where should an attribute of a one to one relationship be placed?

Answer 18

On the relationship set or in either entity set

Question 19

When is inheritance applicable?

Answer 19

• When all potential subclasses share all attributes

- When all potential subclasses share all relationships

Question 20

What are the four steps in the simple modelling methodology?

Answer 20

1. Decide what entity sets are required
2. For each entity set, decide on attributes
3. Decide on relationships between entity sets
4. Decide on generalisation/specialisation