Primary keys uniquely identify records in a table. Foreign keys link tables by providing a link to a related table. A compound key uses two or more attributes to uniquely identify records when no single attribute can be used as the primary key, such as in a table recording bike rentals where a member can rent multiple bikes and bikes can be rented by multiple members, so the compound key uses the member ID, bike ID, and rental date.

1. DATABASE KEYS
Higher Computing Science

2. KEYS
Primary Key
• A primary key is a column whose values uniquely identify a record.
Foreign Key
A foreign key is a column in a relational table that provides link to a
related table.

3. COMPOUND KEY
• A compound key is where two or more attributes are used to uniquely
identify each record in a table.
• Each attribute from the compound key is a primary key from a different
table.
• A compound key is used when no single attribute in a table can be used
as a primary key.

4. COMPOUND KEY EXAMPLE
Members Bikes Hire
MemberID BikeID MemberID*
Name Make BikeID*
Address Colour HireDate
Telephone MT338X440
Two-Wheel-Rental Inc hires bikes to customers for one day. They have a
relational database with three tables as shown below:

5. COMPOUND KEY EXAMPLE
Members Bikes Hire
MemberID BikeID MemberID*
Name Make BikeID*
Address Colour HireDate
Telephone MT338X440
The Hire table needs a compound key of MemberID, BikeID and HireDate.
• MemberID alone can’t be used – a member can make more than one
hire
• BikeID alone can’t be used – a bike can be hired by more than one
member
• MemberID and BikeID can’t be used – a member could hire the same
bike on a different day