A candidate key is a minimal superkey in a relational database, which means it is any set of columns that have a unique combination of values in each row, with the additional constraint that removing any column could produce duplicate combinations of values. In other words, it is a set of one or more columns that can be used to uniquely identify a row within a table. A candidate key can be simple (having only one attribute) or composite (having multiple attributes) . 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.

Candidate keys are an important concept for the design of database schema because they are used to identify the tuples(rows) uniquely in the table. They play a vital role in maintaining the integrity and consistency of a database by ensuring that each row within a table is unique and can be identified independently. A table can have multiple candidate keys, but only one of them can be chosen as the primary key, which is used as the official unique identifier for a row within the table. The value of a candidate key cannot be null, and it must contain unique values.