Entity Relationship Diagrams an Introduction 2

Contents

Contents...... 1 Entity Relationship Diagrams – An Introduction...... 2 Overview...... 2 Reading the ERD...... 3 Key Terms in an Entity Relationship Diagram...... 4 Entity...... 4 Entity Occurrence...... 4 Attributes...... 4 Primary Key...... 4 Relationships between Entities...... 5 One-to-one...... 5 One-to-many...... 5 Many-to-many...... 5 Entity Relationship Diagrams (Cardinality and Optionality)...... 6 Cardinality...... 6 Optionality...... 7 Entity Relationship Diagrams – Many to Many Relationships...... 8 Entity Relationship Diagrams – Selecting Attributes...... 9

1 © Phil James www.philjameswebsite.com Entity Relationship Diagrams – An Introduction

Entity relationship diagrams (ERDs) describe data and the relationships between data items – its semantics or meaning. The construction of these diagrams is known as data modelling.

Overview A company has a number of projects on-going at any time. Each project will be run by a single engineer and over time that engineer will have a number of projects. What entities do we need to keep information on?  Engineer  Project

What attributes (fields) will be needed for each entity?  Engineer - each engineer has a unique identifier (e.g. 100), and a name  Project – a project identifier (e.g. Eng01) and a project title

We need to look at the information that we have and come up with suitable names for these attributes and types:

When an attribute is underlined – it is a primary key. A primary key is a unique identifier for an instance of an entity.

2 © Phil James www.philjameswebsite.com Reading the ERD

The description that we give to the relationships between each entity (and we need to read that relationship both ways:

Engineer -> Project:  An engineer MAY work on one or more projects (dotted line indicates an optional relationship)  An engineer has a one-to-many (many is indicated by the crows foot) relationship with a project

Project -> Engineer:  A project will have an engineer (solid line indicates a mandatory relationship)

3 © Phil James www.philjameswebsite.com Key Terms in an Entity Relationship Diagram

 Entity  Entity Occurrence  Attributes  Primary Key

Entity

Media Media_Number Media_Title Media_Rating Media_Cost_Price Media_Rental_Price 1 Source Code 12 £13.99 £3.00 2 Despicable Me U £6.99 £1.50 3 Mars Attacks 12 £10.99 £2.00

Entity Occurrence

MEDIA - Represents the name of the entity which is transformed into a table.

This is an example of a record also known as an entity occurrence or instance - these are represented in the table as rows.

Media_Number Media_Title Media_Rating Media_Cost_Price Media_Rental_Price 2 Despicable Me U £6.99 £1.50

Attributes

Media_Title (below) is an example of an attribute/field name which is part of an entity.

Attributes are represented in the table as columns.

Media_Title Source Code Despicable Me Mars Attacks

Each of these (Source Code, Despicable Me, Mars Attacks) is an attribute occurrence.

Primary Key

Media_No - the attribute/field name is underlined indicating that it is a primary key. This provides each row with a unique identifier.

4 © Phil James www.philjameswebsite.com Relationships between Entities

 Relationships are verbs (whereas entities are nouns)  Relationships are described by their degree (cardinality)

o One-to-one 1:1

o One-to-many 1:m

o Many-to-many m:n

One-to-one The DVLA should have one tax disc allocated to a car and each car should have a tax disc

One-to-many A mother may have many children but a child can only have one mother

Many-to-many Many items supplied by a supplier and a supplier may/will offer many items

Questions Describe the following degrees of relationship (explain the relationship in both directions and what:

Entity 1 Relationship Entity 2 MP Represents Constituency Department Works in Secretary Councillor Attends Committee Warehouse Contains Product

5 © Phil James www.philjameswebsite.com Entity Relationship Diagrams (Cardinality and Optionality)

Note: Each of these is considered from the direction of A to B.

Cardinality One to One (1:1)

One to Many (1:m)

Many to One (m:1)

Many to Many (m:n)

Mandatory to Optional

Optional to Mandatory

Optional to Optional

Previously A company has a number of projects on-going at any time. Each project will be run by a single engineer and over time that engineer will have a number of projects.  An engineer MAY work on one or more projects (dotted line indicates an optional relationship)  A project will have an engineer

In reality we would more likely have the following situation:

 An engineer MAY work on one or more projects (dotted line indicates an optional relationship)  A project will be run by one or more engineers

Many-to-many relationships are a problem and need to be changed into two one-to-many relationships

In practice the relationship between the two entities becomes in itself an entity – the name (noun selected) of this may be something obvious or it may be more abstract (below the relationship entity is called assignment).

In the diagram above the following attributes are recorded for Engineer:  Engineer_ID  Engineer_FirstName  Engineer_Surname

For Project:  Project_ID  Project_Title

For Assignment:  Assignment_No

9 © Phil James www.philjameswebsite.com As stated last week each entity (table) must have a primary key which uniquely identifies each instance (row) when data is entered. Normally a relationship between two entities is signified not only by a line between the two entities showing cardinality and optionality but also by the inclusion of a foreign key. A foreign key is a field that holds the same value held in the table with the primary key enabling the two tables to be linked and the relationship maintained.

Engineer_ID Engineer_FirstName Engineer_Surname 1 Fred Bloggs 2 John Smith

Project_ID Project_Title C12 Cardboard boxes P01 Plastic Ducks

Assignment_No Engineer_ID Project_ID 1 1 C12 2 1 P01 3 2 P01

Even though the QSEE diagram does not show foreign keys it does maintain them and when the design is converted to SQL it includes these fields – you just cannot see them!