A candidate key is a set of one or more columns that can be used to uniquely identify a row within a table in a relational database model. It is also known as a "unique identifier" or "primary key". The minimal set of attributes that can uniquely identify a tuple is known as a candidate key. A table can have multiple candidate keys, but only one of them can be chosen as the primary key. The primary key is used as the official unique identifier for a row within the table. The value of a candidate key is unique and cannot be null for a tuple. The candidate key can be simple (having only one attribute) or composite (having more than one attribute) . The role of a candidate key is to identify a table row or column uniquely.