Candidate Key

A candidate key is any column (or set of columns) that could legally serve as a table's primary key — it uniquely identifies every row and contains no NULLs. A table can have several candidate keys; one is chosen as the primary key, the rest become unique constraints.

For an employees table, both employee_id and email might each uniquely identify a row — both are candidate keys. Whichever one you designate PRIMARY KEY, the other should still be enforced with a UNIQUE constraint so its uniqueness isn't silently lost.

Candidate keys are a design-time concept from relational theory; constraints are how you enforce a chosen candidate key at the database level.