Introduction to ER Modelling

Question 1

What is the definition of entity

Answer 1

Real-world object distinguishable from other objects.

It is describe using a set of attributes

Question 2

What is the definition of entity set?

Answer 2

A collection of entities of the same type (e.g all employees)
- All entities in an entity set have the same set of attributes
- Each entity has a key (underlined)

Question 3

What is the definition of relationship?

Answer 3

Association among two or more entities
- Relationships can have their own attributes

Example: Fred works in the Pharmacy department

Question 4

What is relationship set?

Answer 4

Collection of relationships of the same type

Example: Employees work in departments

Question 5

What kind of relationship can an entity set participate in?

Answer 5

Different relationship sets

Different “roles” in the same set

Question 6

What is the definition of key constraints?

Answer 6

They determine the number of objects taking part in the relationship set

Question 7

What are the types of key constraints?

Answer 7

Many-to-many

One-to-Many/ Many-to-one

One-to-one

Question 8

What is an example of many-to-many relationship?

Answer 8

An employee can work in many departments, but a department can have many employees

Question 9

How is a many-to-many key represented in Chen Notation?

Answer 9

A line

Question 10

What is a one-to-many relationship?

Answer 10

One-to-many constrains one entity set to have a single entity per a relationship

Question 11

How is a one-to-many key represented in Chen Notation?

Answer 11

An arrow

Question 12

How to identify a weak entity?

Answer 12

A weak entity can be identified uniquely only by considering (the primary key of) another (owner) entity

Question 13

How is a weak entity represented in Chen Notation?

Answer 13

Bold rectangle

Question 14

How is a weak entity’s partial key represented in Chen Notation?

Answer 14

A dashed underline

Question 15

What is an owner entity set?

Answer 15

<Policy> and weak entity set [Dependent] must participate in a relationship where each entity has one and only one strong entity to depend on (key constraint)
</Policy>

Question 16

What is the definition of multi-valued attributes?

Answer 16

Multi-valued attributes can have multiple (finite set of) values of the same type

Example: employees with a home phone number and work phone number

Question 17

What is the definition of composite attributes?

Answer 17

Composite attributes have a structure hidden inside (each element can be of different type)

Example: Employee address consists of postcode, street name and number

Question 18

What are the choices to be made in design phase?

Answer 18

• Should a concept be modelled as an entity or an attribute?
• Should a concept be modelled as an entity or a relationship?
• Should we model relationships as binary. ternary, n-ary?

Question 19

What is an example of Entity vs Attribute

Answer 19

Should “address” be an attribute of Employees or an entity (related to Employees)?

Question 20

What is the justification or variables to the question:

Should “address” be an attribute of Employees or an entity (related to Employees)?

Answer 20

Depends upon how we want to use address information, and the semantics of the data

- If we have several addresses per employee, address must be an entity

- What if an address links to both Employee and WorkFromHomeContract

Question 21

What is participation constraints

Answer 21

explores whether all entities of one entity set take pat in a relationship

Question 22

What is total participation?

Answer 22

Total participation says that each entity takes part in “at least one” relationship and is represented by a bold line

Example: Every employee must work in at least one (Total) department. Each department has at least one(Total) employee

Question 23

What is partial participation?

Answer 23

Partial participation means that not every entity takes part in a relationship

Each department must have one (total) manager (but not everyone is a manager (P =optional)