Relational Databases

• What is a relational database?

• What would we use one for?

• What do they look like?

• How can we describe them?

• How can you create one?

Relational Database

• A relational database is a collection of data items organised as a set of formally-described tables from which data can be accessed or reassembled in many different ways without having to reorganise the database tables (whatis.com definition)

• Or, in other words, a relational database is a database that consists of multiple tables joined together by relationships

Meaningful Information

• Data are stored in databases so that they can be sorted and searched – you could just store data in Word!

• You can use the data in the database to derive meaningful information – for example, there isn’t any data in a library database to tell us which books are overdue, but we can create a query to tell us

• Relational databases also allow things called aggregate functions that work on groups of records and can be used for statistical analysis

Entities and Tables

• When you are designing a database for a particular situation, you need to think about what entities are involved

• Entities are usually things, but could also be an action or process, such as a loan.

• Information for each entity will be stored in its own table in the database

• A table is made up of fields, and each field will have a data type

• Each table also has a primary key – something about the entity that is unique, such as registration number for a car or National Insurance number for a person

There is a standard form for describing tables:

• The NAME of the table appears in upper case

• A list of the fields, separated by commas, appears in brackets after the table name

• The key fields are underlined

For example:

• CAR (registration, make, model, colour)

Standard Database Notation

• Tables are joined together by relationships

• A field that is linked to another table is called a foreign key

• Relationships have a degree:

• One-to-One

• One-to-Many

• Many-to-Many

• We can show these on an entity-relationship diagram...

Relationships

ER diagrams show entities and relationships:

• The entities are shown as rectangles. They are named, and will probably represent the tables you will have in your database

• The relationships are shown as lines, with text above them to describe the relationship

• The degree of the relationship is indicated by the end of the line – plain for “1” and forked for “many”.

Entity-Relationship Diagrams

For example:

Entity-Relationship Diagrams

Book

ISBN

title

author

publisher

Borrower

borrower_id

forename

surname

address

telephone

date of birth

Loan

borrower

book

date taken

date due

date returned

Advantages of Relational Databases

• Data about each of the entities is only entered once:

– There is therefore less duplication (also known as redundancy) of data

– There will be no inconsistencies across the duplicates (as they won’t exist!)

– Less disc space will be required for storing the data

• It can actually make searching easier.

A databases management system, such as Accessshould be able to:

• Store, retrieve and update information in the database in a transparent manner and display it to multiple users

• Implement file and/or record locking to prevent conflicts when data are changed.

• Look after security - at field, table, form, query, etc., level, to prevent unauthorised access

Database Management Systems

The DBMS will:

• Perform back-up and recovery procedures - e.g. repairing or copying database structure

• Allow control of the user interface through forms, reports, buttons and macros

• Queries - “query by example” (e.g. Access queries) or “query language” (e.g. SQL - structured query language)

DBMS continued…