1.Even though both the primary key and unique key are one or more columns that can uniquely identify a row in a table, they have some important differences.
2.Most importantly, a table can have only a single primary key while it can have more than one unique key.
Primary key can be considered as a special case of the unique key.
3.Another difference is that primary keys have an implicit NOT NULL constraint while the unique key does not have that constraint.
4.Therefore, unique key columns may or may not contain NULL values but primary key columns cannot contain NULL values.