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Because databases can get very big, it is important to decide exactly what is going to be stored in each field.
Fields can be text , number , date , time , currency , logical (Yes/No) or look-up (choice from a selected list of words).
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Some databases also allow the user to create an image field . This can be very useful, for example in the student file, as it would allow a photo of each student to be added to each record.
Another type of field is a calculated field . This is usually the result of a calculation based on numbers in another field.
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It is usually a good idea to have a primary key field – a field which is different for every record, such as a customer number.
This helps to identify each record, because there can be no two records with an identical key field.
The database can automatically give each new record a number by setting the field type to autonumber .
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This database shows records of students joining a college.
Can you match the data type to the field name?
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Validation is where the database looks for mistakes in data being entered.
The database checks the data against the validation rules, and if the data is not valid, it gives the user an error message .
Validation cannot prevent all data entry mistakes, but it can find some.
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In our database, one way we could validate the Date of
Birth field is so that it only accepts dates after 1910.
We could validate the Number of GCSE Passes field to be less than 20.
That wouldn’t stop us entering Jerome’s passes as 1 instead of 7, because 1 and 7 would both be valid .
To find that mistake, we would have to verify the data by checking it against the source.
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Format check – e.g. 2 letters then 2 numbers – PR15 would be valid.
Presence check – checking that the field contains data.
Range check – data is between certain numbers or letters, e.g. A –E.
Lookup check – only values from a set list can be entered, e.g. M or F.
Size check – checking data is more or less than a minimum or maximum number of characters.
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If data is entered which breaks the validation rules, the database can give an error message to explain what is wrong.
It’s best if error messages are written in plain language to help the user understand what is wrong. For instance, if a field can only contain the data “M” or “F”, the message:
Entry not valid is not nearly as helpful as:
Not telling!
Please enter M or F
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The college database needs extra fields added. How would you validate them?
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Databases are used in many organizations to store information, for example: schools store information about pupils banks store information about customers the police store information about criminals and cases shops store information about products they sell directory enquiries companies store telephone numbers.
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Forms are designed to be viewed on screen. They make it easier for users to enter data into a database.
Reports are designed to be printed.
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Databases are used because: data can quickly be sorted into order it is quick to search for information, and you can search on any field you can even use a wildcard search if you only know part of the record. For instance, if you are looking for a person’s name, and you know the name begins with with Black , a wildcard search for Black* would return
Black ham and Black stock as well as Black.
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The college library lends Books
Book ID
0001
Title
Fishing
Author Publisher
J P Jones Bookit
0002
0003
Sewing
Football
P R Green
G K Singh
Morebook
Bookit to Students .
Student ID Last name First name
0103 Simons Michael
0104 Chan Peggy
12H
Form
12N
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In a simple flat file database system, when a student wanted to borrow a book, all the information about that book would need to be typed into the student’s record.
That would take ages, and probably create lots of mistakes.
Instead, a relational database can be created. A new table called Loan is added and the tables are then linked together.
Loan
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To make sure we don’t get two students with the same name muddled up, each student is given a unique ID number.
This is called a primary key field .
The same thing needs to happen to the book table, so each book is given a unique ID number.
Student table
Student ID *
Last name
First name
Form
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* primary key field
Book table
Book ID *
Title
Author
Publisher
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The Loan table is linked to the Student table using the
Student ID as the primary key field.
It is also linked to the loan table using Book ID as the primary key field.
Now the information about the book and the student can be looked up from the other tables instead of being entered each time.
Student table
Student ID *
Last name
First name
Form
Loan table
Student ID
Book ID
Date issued
Date due
Book table
Book ID
Title
Author
*
Publisher
* primary key field
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Book ID Title
0002 Sewing
Author Publisher
P R Green Morebook
Student ID Book ID
0104 0002
Date loaned Date due
1/5/04 15/5/04
Student ID Last name First name Form
0104 Chan Peggy 12N
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Data entered Data looked up
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Databases are used to store data.
A database file is made up of records .
Records are made up of fields .
Fields can be text , number , date , time , currency or logical .
Validation checks data entered for mistakes.
Databases can search and sort information.
Forms are viewed on screen, reports are printed.
Relational databases link two or more files using key fields to save having to enter duplicated data.
Databases that don’t have relational links are called flat file databases.
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