Primary & Foreign Keys (Cambridge (CIE) IGCSE ICT)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
Fields & data types
What is a field?
A field is a single piece of data in a table (column)
'Make' is an example of a field in the 'cars' table below
cars
car_id | make | model | colour | price |
|---|---|---|---|---|
1 | Peugeot | 2008 | Red | 24950 |
2 | Mazda | MX5 | Blue | 17995 |
3 | Citroen | DS4 | Black | 21450 |
4 | Ford | Puma | White | 19500 |
What is a data type?
A data type is the type of data that can be held in a field and is defined when designing a table
Examples of common datatypes are:
Numeric - whole/decimal
Character/string - text data
Date/Time
Boolean - true or false values
In the car table above, the following datatypes would be used:
car_id: numeric
make: string
model: string
colour: string
price: numeric
Primary Keys & Foreign Keys
What is a primary key?
A primary key is a unique field that can be used to identify a record in a table
order_id is the primary key for the orders table
customer_id is the primary key for the customers table
What is a foreign key?
A foreign key is a field in a table that refers to the primary key in another table.
A foreign key is used to link tables and create relationships
In the orders table customer_id is a foreign key - it links an order back to the customer that made the order in the customer table
Key database terminology
Term | Definition |
|---|---|
Table | A collection of records with a similar structure |
Record | A group of related fields, representing one data entry |
Field | A single piece of data in a record |
Data type | Type of data held in a field |
Primary key | A unique identifier for each record in a table. Usually an ID number |
Foreign key | A field in a table that refers to the primary key in another table. Used to link tables and create relationships |
Worked Example
A relational database has been developed for a dance club to store information about their members and the styles of dance they practice.
The database contains two tables: Members and Styles
Figure A shows some data from the tables.
Members
MemberID | FirstName | LastName | DateJoined |
|---|---|---|---|
1 | Zarmeen | Hussain | 2024-01-19 |
2 | Fyn | Ball | 2024-02-01 |
3 | George | Johnson | 2024-02-25 |
4 | Ella | Franks | 2024-03-04 |
Styles
StyleID | MemberID | Style | DateStarted |
|---|---|---|---|
1 | 1 | Hip Hop | 2024-01-22 |
2 | 1 | Ballroom | 2024-02-13 |
3 | 3 | Contemporary | 2024-03-01 |
4 | 2 | Street | 2024-03-07 |
(a) State one benefit of using relational databases [1]
(b) State the name of the field from the Members table that is the most suitable to use as the primary key [1]
(c) State the name of the field from the Styles table that is a foreign key [1]
Answers
(a) reduces data redundancy // reduces data inconsistency [1]
(b) MemberID [1]
(c) MemberID [1]
Guidance
Ignore case (for example: memberid)
No mark if inside quotation marks (for example: "MemberID")
No mark if there is an obvious space in response (Member ID)
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