The Entity Relationship(E-R) Model

Question 1

Purpose of E-R Model

Answer 1

E-R diagrams facilitate database
design

Express logical properties of mini-
world of interest within enterprise

Not limited to any particular DBMS

Question 2

Entity

Answer 2

Object that exists and that can be distinguished from other objects

Question 3

Entity Type

Answer 3

Category of entities

Question 4

Entity Set

Answer 4

Collection of entities of the same type that are well defined

Question 5

Entity Instance

Answer 5

Particular noun

Question 6

Entity Type forms

Answer 6

An intension of entity

permanent definition part

Question 7

Entity instances forms a

Answer 7

Extension of entity

All instances that fulfill the definition at the moment

Question 8

Basic Symbols for E-R Diagram

Answer 8

Entity - rectangle
Attribute - oval
Relationship - diamond
Link - line

Question 9

What does the rectangle represent for the entity?

Answer 9

It represents the entity set, not the individual entitity

Question 10

Attributes

Answer 10

Defining properties or qualities of entity type

Question 11

Domain

Answer 11

Set of allowed values for attribute

Question 12

What are some components of an attribute

Answer 12

• May have null values for some entity instances
• May be multi-valued – use double oval on E-R diagram
• May be composite – use oval for composite attribute, with ovals for components
• May be derived – use dashed oval

Question 13

Best way to remember attributes and entities

Answer 13

Attributes - columns

Entities - rows

Question 14

Superkey

Answer 14

Attribute or set of attributes that uniquely identifies an entity

Question 15

Candidate key:

Answer 15

Superkey such that no proper subset of its attributes is also superkey (minimal
superkey – no unnecessary attributes)

Question 16

Primary key

Answer 16

candidate key actually used for identifying entities and accessing records

Question 17

Alternate key

Answer 17

candidate key not used for primary key

Question 18

Composite key

Answer 18

key with more than one attribute

Question 19

Foreign key

Answer 19

Term used in relational model (but not in the E-R model) for an attribute that is primary key of a table and is used to establish a relationship, usually with another table, where it appears as an attribute also

Question 20

Relationships

Answer 20

Connections or interactions between entity sets

Represented by diamond on E-R diagram

Question 21

Relationships type

Answer 21

Category of relationships

Forms intension

Question 22

Relationships set

Answer 22

Collection of relationships of same type that exist a given moment

Forms extension

Question 23

What does set and type form?

Answer 23

Type forms intension; set forms