What is a candidate key?

A candidate key is a minimal set of one or more columns capable of uniquely identifying a row. A table can have several candidate keys.

One candidate key is selected as the primary key, while other candidate keys can be enforced using UNIQUE constraints.

For example, in an Employees table, EmployeeId may be the primary key while a government-issued employee code or another guaranteed-unique business identifier may be an alternate candidate key.