<p>Contents</p><p>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</p><p>1 © Phil James www.philjameswebsite.com Entity Relationship Diagrams – An Introduction</p><p>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. </p><p>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</p><p>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</p><p>We need to look at the information that we have and come up with suitable names for these attributes and types:</p><p>When an attribute is underlined – it is a primary key. A primary key is a unique identifier for an instance of an entity.</p><p>2 © Phil James www.philjameswebsite.com Reading the ERD</p><p>The description that we give to the relationships between each entity (and we need to read that relationship both ways: </p><p>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</p><p>Project -> Engineer:  A project will have an engineer (solid line indicates a mandatory relationship)</p><p>3 © Phil James www.philjameswebsite.com Key Terms in an Entity Relationship Diagram</p><p> Entity  Entity Occurrence  Attributes  Primary Key </p><p>Entity</p><p>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</p><p>Entity Occurrence</p><p>MEDIA - Represents the name of the entity which is transformed into a table. </p><p>This is an example of a record also known as an entity occurrence or instance - these are represented in the table as rows.</p><p>Media_Number Media_Title Media_Rating Media_Cost_Price Media_Rental_Price 2 Despicable Me U £6.99 £1.50</p><p>Attributes</p><p>Media_Title (below) is an example of an attribute/field name which is part of an entity. </p><p>Attributes are represented in the table as columns.</p><p>Media_Title Source Code Despicable Me Mars Attacks</p><p>Each of these (Source Code, Despicable Me, Mars Attacks) is an attribute occurrence. </p><p>Primary Key</p><p>Media_No - the attribute/field name is underlined indicating that it is a primary key. This provides each row with a unique identifier.</p><p>4 © Phil James www.philjameswebsite.com Relationships between Entities</p><p> Relationships are verbs (whereas entities are nouns)  Relationships are described by their degree (cardinality)</p><p> o One-to-one 1:1</p><p> o One-to-many 1:m</p><p> o Many-to-many m:n</p><p>One-to-one The DVLA should have one tax disc allocated to a car and each car should have a tax disc</p><p>One-to-many A mother may have many children but a child can only have one mother</p><p>Many-to-many Many items supplied by a supplier and a supplier may/will offer many items</p><p>Questions Describe the following degrees of relationship (explain the relationship in both directions and what:</p><p>Entity 1 Relationship Entity 2 MP Represents Constituency Department Works in Secretary Councillor Attends Committee Warehouse Contains Product</p><p>5 © Phil James www.philjameswebsite.com Entity Relationship Diagrams (Cardinality and Optionality)</p><p>Note: Each of these is considered from the direction of A to B. </p><p>Cardinality One to One (1:1)</p><p>One to Many (1:m)</p><p>Many to One (m:1)</p><p>Many to Many (m:n)</p><p>6 © Phil James www.philjameswebsite.com Optionality Mandatory to Mandatory</p><p>Mandatory to Optional</p><p>Optional to Mandatory</p><p>Optional to Optional</p><p>7 © Phil James www.philjameswebsite.com Entity Relationship Diagrams – Many to Many Relationships</p><p>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</p><p>In reality we would more likely have the following situation:</p><p> 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</p><p>Many-to-many relationships are a problem and need to be changed into two one-to-many relationships</p><p>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).</p><p>8 © Phil James www.philjameswebsite.com Entity Relationship Diagrams – Selecting Attributes</p><p>In the diagram above the following attributes are recorded for Engineer:  Engineer_ID  Engineer_FirstName  Engineer_Surname</p><p>For Project:  Project_ID  Project_Title</p><p>For Assignment:  Assignment_No</p><p>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.</p><p>Engineer_ID Engineer_FirstName Engineer_Surname 1 Fred Bloggs 2 John Smith</p><p>Project_ID Project_Title C12 Cardboard boxes P01 Plastic Ducks</p><p>Assignment_No Engineer_ID Project_ID 1 1 C12 2 1 P01 3 2 P01</p><p>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!</p><p>10 © Phil James www.philjameswebsite.com</p>