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 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 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…