how to create a unique key over two fields.

I need to create a unique key over two fields, (from and to), both of which are foreign keys. The primary key is offer_id, which is unique.

The reason for doing this is that an 'offer' should never contain the same from AND to numbers, although it is ok to contain the same from OR to numbers.

for example if a from/to of 29/30 exists

29/31 is ok
34/30 is ok
34/29 is ok
29/30 is not ok
30/29 is not ok

How do i set this up in the database?

I am using MySQL phpadmin and have recently downloaded the workbench.

Here the data layout.

-- phpMyAdmin SQL Dump
-- version 3.2.4
-- http://www.phpmyadmin.net
--
-- Host: localhost
-- Generation Time: Feb 09, 2010 at 07:17 PM
-- Server version: 5.1.41
-- PHP Version: 5.3.1

SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";

/*!40101 SET @9272211_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @9272211_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @9272211_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;

--
-- Database: `guitarswap`
--

-- --------------------------------------------------------

--
-- Table structure for table `offer`
--

CREATE TABLE IF NOT EXISTS `offer` (
  `offer_id` int(10) NOT NULL AUTO_INCREMENT,
  `from` int(10) NOT NULL,
  `to` int(10) NOT NULL,
  `date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `status` varchar(10) NOT NULL,
  PRIMARY KEY (`offer_id`),
  KEY `from` (`from`),
  KEY `to` (`to`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=16 ;

--
-- Dumping data for table `offer`
--

INSERT INTO `offer` (`offer_id`, `from`, `to`, `date`, `status`) VALUES
(2, 32, 29, '2010-02-08 19:32:28', 'proposal'),
(3, 31, 29, '2010-02-08 19:31:59', 'proposal'),
(9, 30, 29, '2010-02-08 19:27:30', 'proposal'),
(10, 32, 30, '2010-02-08 19:36:09', 'proposal'),
(11, 30, 29, '2010-02-08 19:40:33', 'proposal'),
(12, 30, 29, '2010-02-08 19:40:34', 'proposal'),
(13, 30, 29, '2010-02-08 19:40:35', 'proposal'),
(14, 30, 29, '2010-02-08 19:40:35', 'proposal'),
(15, 32, 31, '2010-02-08 19:56:48', 'proposal');

--
-- Constraints for dumped tables
--

--
-- Constraints for table `offer`
--
ALTER TABLE `offer`
  ADD CONSTRAINT `offer_ibfk_1` FOREIGN KEY (`from`) REFERENCES `advert` (`advert_id`),
  ADD CONSTRAINT `offer_ibfk_2` FOREIGN KEY (`to`) REFERENCES `advert` (`advert_id`);

/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;