Entity Basics

Entity

Person, place, or thing in which data is collected (noun(

Entity Relationship Diagram

A graphical representation of the ER model, showing entities, attributes, and relationships

Entity Relationship Model

High-level conceptual data model that represents real world entities and their relationships as a graphical schema

Chen Notation

A specific graphical representation in ERDs, entities are rectangles and relationships are diamonds

Crow’s Foot Notation

Graphical representation of CROW feet, showing many sides and whatnot

Implementing ERs

Process of translating ERD components into physical database tables and their linkages

Attribute

Descriptive property or characteristic of an entity

DOB, phone number, name, email

Composite Attribute

Can be divided into small sub attribute

An address can be divided to city, state, and country

Simple Attribute

Cannot be further divided into sub attribute

Name can’t be further divided

Derived Attribute

Can be derived from other attributes

Age is a derived attribute from DOB

Multivalued Attribute

Can contain multiple values

Phone number can have like an emergency contact

Single-Value Attribute

Can’t have more than one value

DOB can’t have multiple birthdays

Optional Attribute

Does not require a value; almost like NULL

Required Attribute

Must have a value

Entity Instance

An individual thing

The employee SAM

Relationship instance

Statement about entity instances

Maria Rodriguez manages Sales

Attribute Instance

An individual value

The salary $35,000

Cardinality

Maxima and minima number of relationships an

Relationship

Association between two or more entities

Connectivity

Describes the type of relationship between entities (Like M:N)

Many-to-Many (M:N))

Where one instance of an entity can be related to multiple instances of another entity, and vice versa

An employee can work on multiple projects, and each project can have multiple employees working on it

One-to-Many

One entity can be associated with multiple entities

A single surgeon department can have many doctors

One-to-One

When each entity in each entity set can take part only once in the relationship

One person can be issued only one passport, and one passport is issued to only one person

Relationship Degree

The number of entities participating in a relationship

Unary, Recursive, Binary, Ternary

Unary Relationship

When there is only ONE entity set participating in a relationship

Recursive Relationship

Nature of; relationship an entity has with itself

Employee manages Employee

Binary Relationship

Relationship between two distinct entities

Ternary Relationship

Relationship that involves three distinct entities

Existence-Independent

An entity can exist without another entity

Existence-Dependent

Entity cannot exist without another entity

Mandatory Participation

Condition where an entity occurrence must be associated with an occurrence of another entity in a relationship

Optional Participation

Condition where an entity occurrence may or may not be associated with an occurrence of another entity in a relationship

Regular Entity

Has a primary key and is existence-independence

Weak Entity

Entity that cannot be uniquely identified by its own attributes and relies on strong entity for identification

Strong Entity

An entity that can exist independently and has its own primary key

Strong (Identifying) Relationship

Relationship where the primary key of the parent entity is included as part of the primary key of the dependent (weak) entity

Weak (Non-Identifying) Relationship

A relationship where the primary key of the parent entity is included as a foreign key in the dependent (weak) entity