G045 Lecture 09 ERD Diagrams (Entity Relationship Diagrams)

Mr C JohnstonICT Teacher

www.computechedu.co.uk

Session Objectives

Understand what an Entity Relationship Diagram is and what they show,

Know the different components of an ERD and how together they can make a diagram,

Know about supporting documentation for ERDs,

Be able to draw ERDs and produce supporting documentation for given scenarios.

Entity Relationship Diagrams

An entity is a person, place, event, or thing for which we intend to collect /store data

University -- Students, Faculty Members, Courses Airlines -- Pilots, Aircraft, Routes, Supplier

Each entity has a number of characteristics known as attributes

Student – Student ID, Name, DOB, Home Address, Aircraft – Aircraft Number, hours flown, last maintenance

date, Entities within an organisation are related to one

another and are shown within a diagram The diagram shows symbolically how the entities relate

to one an other.

Entity Relationships A relationship is a link or associations between entities.

Normally one dentist has many patients

There are three types of relationships: One–to–One (one student has one address) One–to–Many (one student has many grades) Many–to–Many (one student has many students BUT one teacher has many

students)

Due to database rules however we only use: One – to – Many relationships when designing data structures.

Any many-to-many relationships which you discover will need removing.

Entity Relationships Diagrams

EMPLOYEE COMPANY CARdrives

WARD PATIENT holds

ALBUM SINGERS features

This is a way of showing relationships between entities,

Entity names are always written in capitals and appear in rectangles,

Lines join entities together and have: Symbol to show the degree of the relationship, A description to show the name of the relationship,

Many – To – Many Resolve Many - to – Many relationships are not allowed so we

have to resolve them:

SINGERALBUM

SINGER

ALBUM ALBUM / SINGER

features

complied of

To do this we introduce an additional entity between them splitting them up and creating 2 one - to – many relationships.

Quick Exercise1) Draw an entity relationship diagram to show the

relationship between Product and Component (assume company makes 1 component) House and Street Pet Owner and Pet

2) Draw an entity relationship diagram providing a many – to – many resolve to show the relationship between

Product and Component (assume company makes lots of components) Student and A-Levels in a school Stock and Supplier in a newsagent (assume same products are

available from different suppliers)

COMPONENTPRODUCT

PET OWNER PET

STREET HOUSE

COMPONENTPRODUCT PRODUCT / COMPONENT

STOCK SUPPLIERSTOCK / SUPPLIER

STUDENT STUDENT / A-LEVEL A-LEVEL

Supporting Documentation Once the ERD diagram is complete we then need to

produce supporting documentation, Supporting documentation involves a description of

the relationship and thinking about individual data items stored within an entity – (an attribute),

Attributes have a name, a data type, rules, key fields will also be declared.

Attributes will be detailed in a table as shown belowEntity Key Attribute Data type Size

STUDENT

Primary

StudentIDTitleFirst NameSecond NamePost Code

AutoNumberTextTextTextText

Auto4chars15chars20chars8chars

Attribute Data Types Attributes must be one of the following data types:

Text or String, Numeric, Date and Time, Currency, Boolean or Logical, AutoNumber.

Attribute Data Types Text or String:

Consists of a sequence of any characters, Limited to 255 characters, For longer text requirements you can use a memo data type.

Number: Byte: 0 to 255 Integer: -32,768 to 32,768 Long Integer: 2,147,483,648 to 2,147,483,647 Single: small decimal numbers (7dp) Double: medium decimal numbers (15dp) Decimal: large decimal numbers (28dp)

Attribute Data Types Date and Time

General Date – 19/06/1994 17:34:23 Long Date – 19 June 1994 Medium Date – 19 – JUN - 94 Short Date – 19/06/1994 Long Time – 17:34:23 Medium Time – 05:34 PM Short Time – 17:34

Currency Offers more precision for storing prices

Boolean / Logical True, False On, Off Yes, No

Using The Correct Data Types

You must ensure that the correct data type is chosen for an attribute

The larger the maximum value the more memory in the database is taken up

E.g phone numbers don’t need to be calculated with so can be

stored as text

Key Fields Each entity will have a primary key – this is a unique

identifier for each set of data within it, To join one entity to another we use a foreign key, This is the primary key from the entity whose

relationship is one, in the table whose relationship is many

WARD PATIENT holds

wardID patientIDwardID

Quick Exercise1) Produce attribute tables for the relationship between

customer and Videos in a rental shop.

VIDEO VIDEO / RENTAL

GENRE

PRICE BAND

CERTIFCATE

CUSTOMER

RENTAL

Entity Attribute Key Data Type Size

Video VideoIDTitleGenreIDCertificateIDPriceBandID

PK

FKFKFK

AutonumberTextNumberNumberNumber

100 charsIntegerIntegerInteger

PriceBand PriceBandIDPricePerDay

PK AutonumberCurrency

Certificate CertificateIDCertificate

PK AutonumberText 3 chars

Genre GenreIDGenre

PK AutonumberText 20 chars

Video/Rental VideoIDRentalID

FKFK

NumberNumber

IntegerInteger

Rental RentalIDcustomerIDstartDatefinishDate

PKFK

AutonumberNumberDate/timeDate/time

IntegerShort dateShort date

Customer customerIDfirstNamesurNameAddress#1etc

PK AutonumberTextTextText

25 chars30 chars30 chars

Drawing Diagrams Steps to draw an ERD:

Read through the scenario and identify the main entities – (these should be the same as the data stores on the level 1 DFD)

Draw a diagram linking the entities together, Resolve any many to many relationships, Think about any further entities which may exist – remember the

diagram is for the new system so new entities are allowed, Fully describe each entity and its relationship within the diagram Produce an attribute table to show the attributes for each entity

Example

Draw an ERD and Produce supporting documentation for this scenario:An insurance company system takes details from customers which includes personal, car and past claim data. The information acquired is complied and pasted onto an external broker which generates quotes. On return from the broker the best quote is formatted appropriately and sent to the customer.

Answer

Exercise

Draw an ERD and Produce supporting documentation for this scenario:The university and college admission system (UCAS) handles applications to higher education by students in the UK. Students submit data to the system which includes personal details, estimated grades, courses and institutions they want to apply for and a personal statement. A tutor also submits a reference for each student into the system. The system compiles each students application and forwards it onto the institution applied for. Upon receipt the staff at the university scrutinise the application and return to UCAS an offer or rejection – this is then passed onto the student.

Assignment Link By next lesson produce an ERD and supporting documentation for KASBAH NEWS’ New System. You need:

A complete ERDDescription of the entitiesAttribute table