What is the difference between Primary Key and Unique Key?

Question

In my table I have three unique column this can be referred by other table.
Which key I have to use for this.

Answer 1

Both primary key and unique key uniquely identify a record. At higher level we can have is two difference between them.

1. The not null constraint is by default added to primary key, it means, primary key attribute cannot accept null values, whereas, the attribute declared as unique can accept null values. It is the major difference between the two.
2. Secondly, we can have only one primary key in a relation, whereas, multiple attributes can be declared unique at the same time.

Answer 2

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.

Answer 3

Unique key- unique key accept only one null value accept.but unique key use many time in table.
Primary key-Primary key does not accept null value and primary key use only one time in table.