MySQL 聚合函数里面提供了加，平均数，最小，最大等，但是没有提供乘法，我们这里来利用MYSQL现有的GROUP_CONCAT函数实现聚合乘法。
先创建一张示例表：
CREATE TABLE `tb_seq` (`num` int(10) NOT NULL,`seq_type` enum('yellow','green','red') NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;插入示例数据：
insert into `tb_seq`(`num`,`seq_type`)
values (4,'green'),(1,'red'),(3,'green'),(1,'red'),(8,'red'),(4,'yellow'),(8,'red'),(7,'yellow'),(10,'red'),(1,'red'),(1,'red'),(1,'yellow'),(5,'green'),(9,'red'),(1,'yellow'),(6,'yellow');创建基于逗号分隔符的字符串乘法，前提是字符串逗号分隔的都是数字。
DELIMITER $$
USE `t_girl`$$
DROP FUNCTION IF EXISTS `func_multiple`$$
CREATE DEFINER=`root`@`localhost` FUNCTION `func_multiple`(f_nums VARCHAR(1000)) RETURNS DOUBLE(10,2)
BEGIN-- Created by ytt 2014/10/21.DECLARE result DOUBLE(10,2) DEFAULT 1;DECLARE cnt,i INT DEFAULT 0;SET cnt = CHAR_LENGTH(f_nums) - CHAR_LENGTH(REPLACE(f_nums,',','')) + 1;WHILE i < cntDO-- get multiple result.SET result = result * REVERSE(SUBSTRING_INDEX(REVERSE(SUBSTRING_INDEX(f_nums,',',i+1)),',',1));SET i = i + 1;END WHILE;SET result = ROUND(result,2);RETURN result;END$$
DELIMITER ;好了，我们利用我创建的函数以及MYSQL自带的GROUP_CONCAT聚合函数就可以很方便的实现乘法了。
SELECT seq_type,func_multiple(GROUP_CONCAT(num ORDER BY num ASC SEPARATOR ',')) AS multiple_num FROM tb_seq WHERE 1 GROUP BY seq_type;
+----------+--------------+
| seq_type | multiple_num |
+----------+--------------+
| yellow | 168.00 |
| green | 60.00 |
| red | 5760.00 |
+----------+--------------+
3 rows in set (0.00 sec)
