In SQL Server, we have two keys which distinctively or uniquely identify a record in the database. Both the keys seems identical, but actually both are different in features and behaviours.
Primary Key- Primary key can't accept null values.By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.We can have only one Primary key in a table.
Secondary Key-Unique key can accept only one null value.By default, Unique key is a unique non-clustered index.We can have more than one unique key in a table.