unique key constraint in sql server code example

Example 1: unique constraint mssql

USE AdventureWorks2012;  
GO  
CREATE TABLE Production.TransactionHistoryArchive4  
 (  
   TransactionID int NOT NULL,   
   CONSTRAINT AK_TransactionID UNIQUE(TransactionID)   
);   
GO

Example 2: unique key in sql

Unique Key:
Only unique value and also can contain NULL