Entity Relationship Modeling

2Y2 | Midterms

Entity Relationship Modeling Fundamentals

• The Entity Relationship Model (ERM)

is a data model that describes relationships among entities at the conceptual level with the help of ER diagrams.

• The Entity Relationship Diagram (ERD)

is a diagram that depicts an entity relationship model’s entities, attributes, and relations.

• entity

is represented in the ERD by a rectangle, also known as an entity box. The name of the entity, a noun, is written in the center of the rectangle.

• The entity name is generally written in capital letters and in singular form: SINGER rather than SINGERS, and SONG rather SONGS.

• entity instance or entity occurrence

Each row in the relational table is known as an ___ in the ER model.

• entity set

A collection of similar entities is known as an .

• Relationships

describe associations among data.

• connectivity

The type of relationship between entities is called __ and is represented in an ERD by a line.

• active or passive verb

The name of the relationship is usually an ___.

For example, a SINGER sings SONGs.

ER Model Notations

• The different types of relationships can be illustrated using the following notations:

o Chen notation

Relationships are represented by a diamond connected to the related entities through a relationship line. The relationship name is written inside the diamond.

Chen notation example

o Crow's Foot notation: Entities are connected by lines, and symbols at either end describe the cardinality of the relationship between the entities.

Crow's Foot example

Attributes

Each entity consists of a set of attributes that describes particular characteristics of the entity.

• In the original Chen notation, attributes are represented by ovals and are connected to the entity rectangle with a line. Each oval contains the name of the attribute it represents. The entity identifier is mapped as the table's primary key and is underlined.

Attributes example

• In the Crow’s Foot notation, the attributes are written in the attribute box below the entity rectangle. Required attributes are in boldface. PK and FK are used to mark the primary and foreign keys.

Example