make key unique code example

Example 1: mysql set field unique

ALTER TABLE mytbl ADD UNIQUE (columnName);

Example 2: unique constraint mssql

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