mysql unique key
I have a table `items` that holds items that have been select by a user but without a unique or primary key as there no unique records on the table because on every row `userID` and `listID` are not unique (user will surely have more than one item) and `itemID` can be unique/primary as more users might select the same item. What is the correct way to add unique and/or primary key on that table that will make sense?

items table:

    CREATE TABLE IF NOT EXISTS `items` (
      `userID` int(20) NOT NULL,
      `listID` int(20) NOT NULL,
      `itemID` int(20) NOT NULL,
      `qty` int(10) NOT NULL
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

items data:

    INSERT INTO `items` (`userID`, `listID`, `itemID`, `qty`) VALUES
    (12, 277, 44, 2160),
    (12, 277, 3683, 13500),
    (12, 277, 3689, 2700),
    (12 277, 9832, 4320),
    (12 277, 9848, 540),
    (12, 277, 16272, 81000),
    (12, 277, 16273, 81000),
    (12, 277, 16274, 243000),
    (71, 278, 12066, 4),
    (82, 279, 44, 2880),
    (82, 279, 3683, 18000),
    (82, 279, 3689, 3600),
    (82, 279, 9832, 5760);


mysql unique key
===

I have a table `items` that holds items that have been select by a user but without a unique or primary key as there no unique records on the table because on every row `userID` and `listID` are not unique (user will surely have more than one item) and `itemID` can be unique/primary as more users might select the same item. What is the correct way to add unique and/or primary key on that table that will make sense?

items table:

    CREATE TABLE IF NOT EXISTS `items` (
      `userID` int(20) NOT NULL,
      `listID` int(20) NOT NULL,
      `itemID` int(20) NOT NULL,
      `qty` int(10) NOT NULL
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

items data:

    INSERT INTO `items` (`userID`, `listID`, `itemID`, `qty`) VALUES
    (12, 277, 44, 2160),
    (12, 277, 3683, 13500),
    (12, 277, 3689, 2700),
    (12 277, 9832, 4320),
    (12 277, 9848, 540),
    (12, 277, 16272, 81000),
    (12, 277, 16273, 81000),
    (12, 277, 16274, 243000),
    (71, 278, 12066, 4),
    (82, 279, 44, 2880),
    (82, 279, 3683, 18000),
    (82, 279, 3689, 3600),
    (82, 279, 9832, 5760);

