Ch.5: Introduction to databases
Theory Notes
Chapter 5
Introduction to databases
Eng. Omar El Safty
Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
5.1 Data types
1. Numeric data
Numeric data simply means numbers. Numbers come in a variety of different types:
Integer
• Positive or negative whole number
• Examples: 12, 45, 1274, 1000000, -3, -5735
Real/Decimal
• Positive or negative numbers with decimal/fractional parts
• Examples: 1, 1.4534, 946.5, -0.0003, 3.142
Currency
• Currency refers to real numbers that are preceded by a currency symbol
• Examples: £12.45, -£0.01, €999.00, $5500.
Percentage
• Percentage refers to any number that is followed by a percent symbol
• Examples: 100%, 25%, 1200%, -5%.
2. Alphanumeric/Text data
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Alphanumeric data refers to data made up of letters and numbers.
Examples: DOG, "A little mouse", ABC123, enquiries@bbc.co.uk.
3. Boolean data
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Boolean data can only have two values.
Examples: TRUE and FALSE, YES and NO, ON and OFF, Male and Female.
4. Date/Time data
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Date/Time data is usually formatted in a specific way according to the setup of the
computer and the user’s preferences.
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Date Examples: 25/10/2007, 12 Mar 2008, 10-06-08.
Time Examples: 11 am, 15:00, 3:00 pm, 17:05:45.
Eng. Omar El Safty
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Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
Selecting data types
When we input data to a computer, we must analyze it and select appropriate data types for each
value.
Student Name:
Ben Smith
Alphanumeric
Student Number:
1234
Numeric Integer
Date of Birth:
10 July 1998
Date/Time
Year Group:
6
Numeric Integer
Telephone:
(0232) 744 6574
Alphanumeric
Special Diet:
Yes
Boolean
Exam Score:
78%
Numeric Percentage
Height:
1.67
Numeric Real
Fees Paid:
$ 1500
Numeric Currency
The data type of a telephone number is Alphanumeric data as:
• Telephone numbers often have spaces, dashes or any other
symbol.
• Telephone numbers may start with a zero (numeric data
cannot start with a zero).
• Telephone numbers are not used in arithmetic operations
Eng. Omar El Safty
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Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
5.2 Definition of database
A database is an organized collection of data related to each other.
5.3 Database terminologies
Term
Definition
Examples
Table
A collection of related records
Table of students
For a student the fields
could include:
Field
Field Name
Record
A column that contains one specific piece of
information and has one data type
•
Student name
•
Student ID
•
Age
•
Address
•
Gender
Title given to each field
A row within a table that contains data about a
Details (all information) of
single item, person or event
one student
Primary Key
Eng. Omar El Safty
45
Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
Primary keys
Definition and features/characteristics:
•
•
•
•
A primary key is a field that contains unique values
Helps identify the record
A primary key cannot be a null value
Creates relationships
5.4 Database types
Flat-file database
Definition:
Database where all the data is held in a single table.
Example:
Relational database
Definition:
•
•
•
Contains more than one table linked to each other
It uses relationships
It makes use of foreign key
Example:
For this example, all the data about customers is held in one file, and all the data about jobs is
held in another file. To create a connection between the two files, a relationship must be created
Eng. Omar El Safty
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Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
Foreign keys
Definition and features/characteristics:
•
•
•
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It is a primary key in another table
Enforces referential integrity
A foreign key can have a null value
Found in relational databases
Example:
'Customer ID' field acts as the primary key in the customer file and the foreign key in the job
file, thus enabling a relationship between the two tables.
Relationships
A relationship is a link or association between fields.
The relationship between the tables can be:
• one-to-one
• one-to-may
• many-to-many
Similarities between relational databases and flat file databases:
•
•
•
Both use a primary key
Both can create reports
Both store data in records and fields in the table
Eng. Omar El Safty
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Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
Advantages of a relational database over a flat-file database:
•
•
•
•
•
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Uses data integrity, which means no inconsistency in the data
No duplication of data, so less data entry
Saves time when entering data, as the user only has to input the data once
Fewer errors are produced due to less data entry
Saves storage space as data is only stored once
Easier to add/modify/delete data
Complex reports can be created
Complex queries can be carried out
Better security as some tables can be made confidential
Disadvantages of a relational database over a flat-file database:
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Takes more time to set up
More complex than flat-file databases as more tables are used
More complex to understand
Sorting/Filtering of data is more complex
Slower extraction of data
Needs more processing power for complex queries
Needs specialist personnel to set up the database
Differences between Primary Key and Foreign Key:
•
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Each table has one primary key, whereas a table can contain several foreign keys
The primary key holds unique data, while the foreign key can be replicated in the table
The primary key identifies the record, while the foreign key is used to link with the
primary key of another table
Eng. Omar El Safty
48
Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
5.5 Data validation
Definition and purpose:
•
•
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An automated computer check
Ensures that the data entered into a computer is sensible/reasonable
Ensures the data entered fits the set criteria
Validation check
Description
Example(s)
In an online form, a person’s
Presence check
Checks if data is present and hasn’t
been left blank
telephone number may be a
required field. So, if no data is
entered, an error message will be
produced.
When registering a course in
Lookup check
Checks if value entered is available
IGCSE, the session must be either
in a limited set/list of items
June, November or Jan. No other
entry should be accepted.
•
Range check
Checks whether numeric data is
within given/acceptable values
•
Uses two boundaries
•
Checks whether numeric data is
Limit check
within given/acceptable values
•
Uses one boundary only
To check if a student’s age is < 18
and > 0
To check if student’s age is < 18
(no lower boundary)
If a field needs six digits, then
Length check
Checks if the input data contains
inputting a five-digit or seven-digit
the required number of characters
number should cause an error
message.
Type check
Format check
Eng. Omar El Safty
Checks that the input data has the
appropriate data type
A person’s age should be numeric
integer not alphanumeric, for
example.
Checks the data is in a specific
A date which should be in the form
format or fixed pattern
of dd/mm/yyyy, for example.
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Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Character check
Theory Notes
It checks that a string of characters
does not contain any invalid
characters or symbols.
•
A name would not contain
characters such as %
•
A telephone number would
only contain digits
A validation check used in barcodes
to validate the correctness of the
Check digit
barcode.
This is done by adding an extra digit
that is calculated from the other
digits using some formula.
Validating the correctness of a
barcode
5.6 Data verification
Definition and purpose:
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Checks that the data on the original source document is identical to the data that you
have entered into the system
•
Ensures that the data is copied/entered correctly
Visual verification
Definition:
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Data is compared visually with the source document by a user
The user is asked to confirm that the data entered is the same as the original
If the user finds a difference, the data is re-entered
Double data entry
Definition:
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Data is entered twice by the same person // by two different people
A computer checks that both entries are equal
If they are not equal, an error message requesting to re-enter the data is displayed
Eng. Omar El Safty
50
Eng. Mustafa El-Komy
Ch.5: Introduction to databases
Theory Notes
Similarities between visual verification and double data entry:
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Both are used to check that data has been entered correctly
Both ensure that the data has been transferred correctly
Both are used to check the data against the original data
Differences between visual verification and double data entry:
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In visual verification the user compares the data, whilst in double data entry the
computer compares the data
•
In visual verification the data is compared with the original data source visually, whilst in
double data entry the data is compared with the previously entered data
•
In double data entry, data is entered twice by the user(s)
Double data entry is different from Proofreading!
Proofreading is reading through the document to check grammar and
spelling mistakes without referring to the original source document.
Eng. Omar El Safty
51
Eng. Mustafa El-Komy