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。
CREATE TABLE `tb_seq` ( `num` int(10) NOT NULL, `seq_type` enum('yellow','green','red') NOT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
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');
DELIMITER $$ USE `t_girl`$$ DROP FUNCTION IF EXISTS `func_multiple`$$ CREATE DEFINER=`root`@`localhost` FUNCTION `func_multiple`( f_nums VARCHAR(1000) ) RETURNS DOUBLE(10,2) BEGIN -- Created by ytt 2014/10/21. DECLARE result DOUBLE(10,2) DEFAULT 1; DECLARE cnt,i INT DEFAULT 0; SET cnt = CHAR_LENGTH(f_nums) - CHAR_LENGTH(REPLACE(f_nums,',','')) + 1; WHILE i < cnt DO -- get multiple result. SET result = result * REVERSE(SUBSTRING_INDEX(REVERSE(SUBSTRING_INDEX(f_nums,',',i+1)),',',1)); SET i = i + 1; END WHILE; SET result = ROUND(result,2); RETURN result; END$$ DELIMITER ;
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)
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