CREATE TEMPORARY TABLE `tmpCart2UserMap` (
 `cartId` int(11) NOT NULL AUTO_INCREMENT,
 `userId` int(11) NOT NULL,
 PRIMARY KEY (`cartId`),
 KEY `userId` (`userId`)
);

INSERT INTO `tmpCart2UserMap`
SELECT MAX(`id`), `userId`
FROM `mm_Cart`
WHERE `userId` > 0 AND `transactionId` IS NULL
GROUP BY `userId`;

SELECT * FROM `tmpCart2UserMap`;

DELETE `c`
FROM `mm_Cart` AS `c`
LEFT JOIN `tmpCart2UserMap` AS `map`
  ON  `map`.`cartId` = `c`.`id`
  AND `map`.`userId` = `c`.`userId`
WHERE
  `c`.`userId` > 0
  AND `transactionId` IS NULL
  AND `map`.`cartId` IS NULL;