The basic differences between Primary Key and Unique key are as follows

 1) By default Primary Key will generate Clustured Index whereas Unique Key will Generate Non-Clustured Index.

2) Primary Key is a combination of Unique and NOT NULL Constraints so it can’t have duplicate values or any NUll Whereas for Oracle UNIQUE Key can have any number of NULL whereas for SQL Server It can have only one NULL

3) A table can have only one PK but It can have any number of UNIQUE Key.