SELECT NEWID()

INSERT INTO Employees
(EmployeeID, Name, Phone)
VALUES
(NEWID(), 'harshit vyas', '99-99999')

to generate a random unique identifier in using sql query in SQL server and to use as unique key 

GUID indicating a Globally Unique Identifier. It consists of a 16-byte hexadecimal numbe. The GUID is useful when a row must be unique among many other rows.