alter table unique code example

Example 1: mysql alter table set column unique

ALTER TABLE contacts
ADD CONSTRAINT contacts_unique UNIQUE (reference_number);

Example 2: unique constraint mssql

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