Week 2 (Chapter 4 - Entity Relationship Modelling)

Question 1

What is an entity? How is it represented in an ERD?

Answer 1

This is an object of interest to the end user. Refers to the entity set and not to a single entity occurency.

Represented by a single rectangle with the entity’s name, name is in all caps AND SINGULAR ONLY

E.g. A table is an entity, a row is not an entity.

Question 2

What is an entity occurence/instance?

Answer 2

This is a single aspect of an entity.

E.g. An entity occurence of a table is a row.

Question 3

What are attributes?

Answer 3

These are the characteristics that describe an entity.

E.g. An entity, student, has attributes such as first/last name, initials, email, phone number etc.

Question 4

What is the difference between required and optional attributes?

Answer 4

A required attribute MUST have a value (cannot be empty)

An optional attribute CAN have empty values.

Question 5

What is the difference between required and optional attributes?

Answer 5

A required attribute MUST have a value (cannot be empty)

An optional attribute CAN have empty values.

Question 6

What are domains?

Answer 6

These are the set of possible values of a given attribute.

E.g. height range, GPA, gender etc.

NOTE: Attributes can share domains.

Question 7

What are identifiers?

Answer 7

These are attributes that uniquely identify each entity instance.

E.g. A primary key is an identifier.

Question 8

What is the difference between composite and simple attributes?

Answer 8

Composite attributes is an attribute that can be subdivided to yield additional attributes

Simple attributes cannot be subdivided.

E.g. An address is a composite attribute (post code, street name, street number, city, state). However, sex or first name are simple attributes.

Question 9

What is the difference between a single-valued and a multi-valued attribute?

Answer 9

Single-valued attributes can only have ONE value.

A multi-valued attribute can have multiple values.

A product’s serial number can only be one value therefore it is single-valued. A multi-valued attribute can incude car colours, heights, material type etc.

Question 10

What are derived attributes?

Answer 10

These are attributes that have a value calculated from other attributes. Not stored, rather calculated with an algorithm.

Question 11

What are derived attributes?

Answer 11

These are attributes that have a value calculated from other attributes. Not stored, rather calculated with an algorithm.

Question 12

Steps of data base design?

Answer 12

1. Conceptual
2. Logical
3. Physical

Question 13

Can an entity ahve multiple keys?

Answer 13

NO, only a single key per entity

can have composite which is a single key created from 2 or more attributes

Question 14

Are linking tables considered entities?

Answer 14

NO

Question 15

Do you specify primary/foreign keys during the conceptual design stage?

Answer 15

No, we DO NOT assume the database type in this stage

Question 16

What makes a strong entity?

Answer 16

An entity that has a key which does NOT need reference to other entities

i.e. has its own unique primary key

Question 17

What makes a weak entity?

Answer 17

If it has a key that requires the existence of one or more other entities

i.e. An entity that requires a composite key for identification

Question 18

Can surrogate keys be added during conceptual modelling?

Answer 18

NO

Question 19

How do you show that an relationship is identifying?

Answer 19

Line between entities is solid.

(one entity points to a weak entity).

Question 20

How do you show that a relationship is non-identifying?

Answer 20

Line between entities is dotted.

BOTH strong entities

Question 21

What are derived attributes?

Answer 21

These are attributes that can be calculated (using an algorithm)

Question 22

When can you add entities during conceptual modelling?

Answer 22

When trying to resolve multi-valued attributes can be shown as a one-to-many relationship. between the parent (original entity) and the child (the multi-variable attribute)

Question 23

What type of entity is a linking/junction table (shows many-to-many relationships)

Answer 23

Associative or Composite or Bridging Entity.