A database is a (large) collection of data items and links between them, structured in a way that allows it to be accessed by a number of different application programs. The term is also used loosely to describe any collection of data. In the past these may have been paper based but nowadays most will be computerised.

Databases are very widely used and many of us will have interacted with one – without necessarily being aware of it. If you have entered your personal details into a social networking site, they have almost certainly been stored in a database; if you have browsed an online retail site, product details will almost certainly have been retrieved from a database. 

Here are some other contexts in which a database might be used: 

• Your school might have a database that contains data about pupils, attendance, subjects and assessment marks; 

• An employer might have a database that contains data about employees, job titles and salaries; 

• The organisers of the Olympic Games might have a database that contains data about athletes, events and medals. 

• Businesses might have databases about their customers, staff and suppliers.

The most common way to structure a database is as a collection of interrelated tables. This kind of database is known as a relational database. 

A table is the name for each group of similar data with rows for each instance of an entity and columns for each attribute. We use a different table for each object in our database and then we link them together. 

An entity is is simply an object that we hold data about and an attribute is a piece of information about that entity.

For example in a database about cars, the enity is car, the attributes would be the data we hold about the car, for example brand, model, colour, number of doors, miles on clock. An instance of an object would be the details for one particular car, for example Vauxhall, Insignia, Black, 4, 40,000.

There are a range of datatypes available to us when creating our database. This allows us to store data in the most suitable way.

Number- Allows us to store numerical values

Text- Allows us to store text information (Short/Long)

Yes/No- Allows us to store information with a check box to indicate yes/no

Date/Time- Allows us to store information about dates and times.

Currency- Allows us to store information relating to monetary values

Look-Up Wizard- Allows us to provide a pre-defined list of options, often used for title e.g. Mr/Mrs etc...

A Record is the basic unit of data stored in a data file. It is a collection of data items, which may be of different data types, all relating to the individual or object that the record describes and is treated as a unit for processing. In our previous example it would be all of the information about the car. A record is made up of many fields.

A field is one single piece of information about an object. It is another word for attribute as explained previously.

A primary key is a field that uniquely identifies an individual record in a table. For example if we have a table containing information about students, what information might we collect? Can any of these fields be used to uniquely identify each record? In some cases we will have to create an additional field such as candidate number to uniquely identify each record.

When the primary key from one table appears as a field in a second table, it is known as a Foreign Key of the second table. 

We use entity relationship diagrams to show how different tables in our database are related.

Create an Entity-Relationship Diagram for the car rental database you have been working on.

To create a relationship we move the primary key from one table to another, where it is known as a foreign key. This piece of data can be used to retrieve all of the other information about an object if necessary.

There are three different types of relationships possible between objects.

1)One to One- A husband can have one wife, a wife can have one husband.

2)One to many- A football team can have many players, a player can play for one team.

3)Many to many- A subject can have many students, a student can have many subjects.

A query is a question used to retrieve selected information from a database. 

For example we might want to ask the following questions in relation to our car database

• Find all of the Ferraris.

• Find all black cars.

• Find all cars with less than 5000 miles.

Special computer languages, called query languages, are used to specify queries in a form that can be processed by a computer. A special piece of software called a query processor analyses the query and computes the answer. 

= will find all matches of this numerical value

> will find all numerical values above this value

<will find all numerical values below this value

We can create queries on one or more fields by adding additional query criteria. We can also use or within a query to return any record with either value. For example everyone called "Sam" or "Fred" or everyone from "Sion" or "Omagh".

We can perform a query on a date. If we want to find someone born on an exact date we type in the full date 1/7/1999. To find everyone born in a particular month  we would enter */7/* and similarly to find everyone born in a particular year we would use */*/1998. The * means this part of the date will be ignored when the query is running.

When querying a "Yes/No" Field we simply enter "yes" of "no"

A report is the presentation of selected data from a database.

Query results are not very presentable particularly when printed. We use a report to make the query results more visually appealing. 

Once we have created a table we can also create a form to aid data entry. This improves the visual appearence of the screen and also will decrease the likelihood of data entry errors.

A macro is a small program to perform a repetitive task and which can be created and stored for later use by a user. This can cut down the amount of time the user spends clicking on options on a menu and assign all of those selections to one simple button. These can be used to print out pages or even link to other pages/documents.

This is a check made to ensure that data being entered to a database is valid, this will help keep our information accurate. We can check the data in a number of ways including...

Presence Check: This checks that some data has actually been entered into a field. If no data is entered the user will be given a message explaining that certain data is missing.

Range Check: This checks that a numeric value is entered which is in  a certain range. For example we could use it to check the age entered by a user is under 110. We could also use it to check all users are over 18.

Type Check: This checks that the data entered is a certain type for example if we ask the user for their date of birth and they enter "bananna" the sime will not accept the data and ask the user to enter it again.

Length Check: This checks the length of a piece of text. For example, to ensure a post code is 7 characters long.