在mysql中统计每人每天num总和

表结构：

DROP TABLE IF EXISTS `countform`;

CREATE TABLE `countform` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) DEFAULT NULL,
  `num` int(11) DEFAULT NULL,
  `time` datetime DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

插入值
-- ----------------------------
-- Records of countform
-- ----------------------------
INSERT INTO `countform` VALUES ("1", "张三", "1", "2015-06-06 10:03:49");
INSERT INTO `countform` VALUES ("2", "张三", "2", "2015-06-06 10:04:09");
INSERT INTO `countform` VALUES ("3", "李四", "22", "2015-06-06 10:06:27");

INSERT INTO `countform` VALUES ("4", "张三", "1", "2015-06-07 10:06:41");

统计每人每天num总和：

select sum(num),a.time,a.username from 

(

SELECT
date_format(time, "%Y-%m-%d") time,num,
username
FROM
countform
GROUP BY
date_format(time, "%Y-%m-%d"),username,num

) a

 group by time,username