25. Difference between Primary Key and Unique Key?

Primary Key	Unique Key
Primary Key can't accept null values.	Unique key can accept only one null value.
By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.	By default, Unique key is a unique non-clustered index.
We can have only one Primary key in a table.	We can have more than one unique key in a table.
Primary key can be made foreign key into another table.	In SQL Server, Unique key can be made foreign key into another table.

Define Primary key and Unique key

CREATE TABLE Employee
(
EmpID int PRIMARY KEY, --define primary key
Name varchar (50) NOT NULL,
MobileNo int UNIQUE, --define unique key
Salary int NULL
)

In one of the differences, I have used 'clustered' and 'non clustered index'. In order to understand the concept, please follow this link:  http://cracksoftwaretestinginterviews.blogspot.in/2014/01/26-clustered-and-non-clustered-index.html