Primary key vs. Unique key
Primary key : It is having null values, initial value is not checked in the checkbox.
Unique key : It does not have null values, initial value is checked in the checkbox.
Primary key exists in database table while unique key exists in internal tables. We can perform actions like sorting, searching, modifying, updating on the basis of primary key in the database table. Same actions can be performed with unique key in the internal tables.
Internal tables with a unique key cannot contain duplicate entries. The uniqueness depends on the table access method.