A candidate key, or simply a key, of a relational database is a minimal superkey. In other words, it is any set of columns that have a unique combination of values in each row (which makes it a superkey), with the additional constraint that removing any column could produce duplicate combinations of values (which makes it a minimal superkey). Because a candidate key is a superkey that doesn't contain a smaller one, a relation can have multiple candidate keys, each with a different number of attributes.

Specific candidate keys are sometimes called primary keys, secondary keys or alternate keys.

The columns in a candidate key are called prime attributes, and a column that does not occur in any candidate key is called a non-prime attribute.

Every relation without NULL values will have at least one candidate key: Since there cannot be duplicate rows, the set of all columns is a superkey, and if that isn't minimal, some subset of that will be minimal.

There is a functional dependency from the candidate key to all the attributes in the relation.

The superkeys of a relation are all the possible ways we can identify a row. The candidate keys are the minimal subsets of each superkey and as such, they are an important concept for the design of database schema.