What is unique key in database?

A unique key is a set of one or more than one fields columns of a table that uniquely identify a record in a database table and the primary key which it can accept one null value but it cannot have duplicate values. The unique key and primary key that they both provide an guarantee for uniqueness for a  column or set of columns.

• There is an automatically defined unique key constraint within a primary key constraint.

• There may be many unique key constraints for one table, but only one PRIMARY KEY constraint for one table.

• The unique key which shows the table that primary key is there and in which the uniqueness is shown in the first column