一個表，表1，ref（唯一值），type（有重複）如何去掉重複記錄。

建表sql語句：

-- ----------------------------
-- Table structure for `deletechongfu`
-- ----------------------------
DROP TABLE IF EXISTS `deletechongfu`;
CREATE TABLE `deletechongfu` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ref` int(11) DEFAULT NULL,
  `type` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;


-- ----------------------------
-- Records of deletechongfu
-- ----------------------------
INSERT INTO `deletechongfu` VALUES ('1', '1', '2');
INSERT INTO `deletechongfu` VALUES ('2', '2', '4');
INSERT INTO `deletechongfu` VALUES ('3', '3', '4');
INSERT INTO `deletechongfu` VALUES ('4', '4', '5');
INSERT INTO `deletechongfu` VALUES ('5', '5', '5');
INSERT INTO `deletechongfu` VALUES ('6', '6', '5');
INSERT INTO `deletechongfu` VALUES ('7', '7', '7');
INSERT INTO `deletechongfu` VALUES ('8', '8', '7');
INSERT INTO `deletechongfu` VALUES ('9', '9', '5');

實現方法如下：

DELETE FROM deletechongfu WHERE id not in(SELECT a.id FROM (SELECT id FROM deletechongfu GROUP BY type) a);