Answers for "how to set unique key in sql"

unique constraint mssql

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

alter unique key sql query

ALTER TABLE Product ADD CONSTRAINT AK_Product_ProductCode UNIQUE( ProductCode )
ALTER TABLE Product ADD CONSTRAINT AK_Product_ProductStamp UNIQUE( ProductStamp )

primary key and unique key in sql

What is the difference between UNIQUE and PRIMARY KEY constraints? 
There should be only one PRIMARY KEY 
in a table whereas there can be
any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL
values whereas Unique key allows NULL values.

unique key in sql

Unique Key:
Only unique value and also can contain NULL