Editing Relational model (section)

=== Database ===
An idealized, very simple example of a description of some [[relvar]]s ([[relation (database)|relation]] variables) and their attributes:
* Customer ('''<u>Customer ID</u>''', Name)
* Order ('''<u>Order ID</u>''', <u>Customer ID</u>, <u>Invoice ID</u>, Date)
* Invoice ('''<u>Invoice ID</u>''', <u>Customer ID</u>, <u>Order ID</u>, Status)

In this [[design]] we have three relvars: Customer, Order, and Invoice. The bold, underlined attributes are ''[[candidate key]]s''. The non-bold, underlined attributes are ''[[foreign key]]s''.

Usually one [[candidate key]] is chosen to be called the [[primary key]] and used in [[preference]] over the other candidate keys, which are then called [[alternate key]]s.

A ''candidate key'' is a unique [[identifier]] enforcing that no [[tuple]] will be duplicated; this would make the [[Relation (database)|relation]] into something else, namely a [[bag (mathematics)|bag]], by violating the basic definition of a [[set (mathematics)|set]]. Both foreign keys and superkeys (that includes candidate keys) can be composite, that is, can be composed of several attributes. Below is a tabular depiction of a relation of our example Customer relvar; a relation can be thought of as a value that can be attributed to a relvar.