Compound Keys
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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.
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Compound Keys
- 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