1. 2.3 Organising Data for Effective Retrieval
Describe the nature and purpose of a relational database, and how it works
Understand how data can be retrieved to produce meaningful information
Recall the advantages of relational database over flat-file systems
Select and justify appropriate file and database structures for particular applications

2. 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

3. 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

4. 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.

5. Database Management Systems
A databases management system, such as Access
should 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

6. DBMS continued… 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)

7. Standard Database Notation
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)

8. Relationships 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!

9. Entity-Relationship Diagrams
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”.

10. Entity-Relationship Diagrams
For example:
Borrower
borrower_id
forename
surname
address
telephone
date of birth
Loan
borrower
book
date taken
date due
date returned
Book
ISBN
title
author
publisher

11. Entity-Relationship Diagrams
Some people also show whether records are required:
Borrower
borrower_id
forename
surname
address
telephone
date of birth
Loan
borrower
book
date taken
date due
date returned
Book
ISBN
title
author
publisher
Optional
Mandatory