Week 3

Question 1

What is Multiplicity?

Answer 1

Number of possible occurences of an entity type that may relate to a single occurrence of an associated entity type through a particular relationship

Question 2

What are examples of Binary Relationships?

Answer 2

One-to-One (1:1)

One-to-Many (1:*)

Many-to-Many (:)

Question 3

What does 0..1 mean?

Answer 3

0 or 1 entity occurence

Question 4

What does 1..1 (or just 1) mean?

Answer 4

Exactly 1 entity occurence

Question 5

What does 0..* (or just *) mean?

Answer 5

Zero or many entity occurences

Question 6

What does 1..* mean?

Answer 6

One or many entity occurences

Question 7

What does 3..7 mean?

Answer 7

Minimum of 3 up to maximumm of 7 entity occurences

Question 8

What does 1, 5, 10-12 mean?

Answer 8

1 or 5 or 10 to 12 entity occurences

Question 9

What is Cardinality?

Answer 9

Maximum values for the multiplicitiy ranges on either side of the relationship

One-to-One (1:1)

One-to-Many (1:*)

Many-to-Many (:)

Question 10

What is Participation?

Answer 10

Minimum values for the multiplicity ranges on either side of the relationship

Optional participationn (if 0)

Mandatory participation (if 1 or more)

eg. participation of 1..* is mandatory as 1 is the minimum

Question 11

What is a Fan Trap?

Answer 11

May exist where two or more 1:* relationships fan out from the same entity

A fan trap in databases happens when one item is connected to multiple related items, making it confusing to aggregate data correctly

Fixed by changing structure of the database

Question 12

What is a Chasm Trap?

Answer 12

A chasm trap in databases occurs when there’s a gap in relationships between entities, making it hard to connect data correctly

Fixed by changing the structure of the database

Question 13

What is the difference between a Fan Trap and a Chasm Trap?

Answer 13

Fan Traps: Path between certain entity occurences is ambiguous

Chasm Traps: Pathway does not exist between certain entity occurences - pathway connection missing

Question 14

Steps for Conceptual Database Design

Answer 14

1. Identify entity types
2. Identify relationship types
3. Identify and associate attributes with entity and relationship types
4. Determine primary keys for entity types
5. Identify Multiplicity constraints
6. Consider use of enhanced modeling (optional)
7. Check model for redundancy traps
8. Validate conceptual model against user transactions
9. Review conceptual data model with user

Question 15

What is an example of a One-To-One (1:1) Relationship?

Answer 15

Staff ID ————–> name
1..1 manages 0..1

Exactly one professor is needed to manage a discipline

Professor could manage at most 1 discipline. Not all professors do management therefore 0..1

Question 16

What is an example of a One-To-Many (1:*) Relationship?

Answer 16

staffNo ————–> studentNo
1 Tutors 0..*

One staff member tutors 0 to many students

Question 17

What is a Many-To-Many (:) Relationship?

Answer 17

newspaperName ———> PropertyForRent
Advertises
0..* 1..*

Each property is advertised in zero or more newspapers

Each newspaper may advertise one or more properties for rent

Question 18

What is Specialisation?

Answer 18

Process of maximising the differences between members of an entity by identififying their distinguishing characteristics

Question 19

What is Generalisation?

Answer 19

The process of minimizing differences between entities by identifying their common characteristics

Grouping entities with common attributes into one super-type

Subtypes of a super-type don’t need to have their primary key displayed in their attributes as its displayed in the super-type

Question 20

What are the 2 types of Participation Constraint?

Answer 20

Optional

Definition: An entity may participate in a relationship, but it’s not required

Example: An employee may have a car but doesn’t have to

Mandatory

Definition: An entity must participate in a relationship

Example: A student must have an enrollment in a course

Question 21

What are the 2 types of Disjoint Constraint?

Answer 21

And - If there is overlaps between roles

Or - if there is not overlaps between roles

Question 22

What is an example of a Specialisation/Generalisation constraint?

Answer 22

{Optional, And}

First part is the Participation Constraint

Second part is the Disjoint Constraint

Question 23

What is Aggregation?

Answer 23

An abstraction through which relationships are treated as higher-level entities