利用MySQL 的GROUP_CONCAT函数实现聚合乘法

MySQL 聚合函数里面提供了加，平均数。最小，最大等，可是没有提供乘法，我们这里来利用MYSQL现有的GROUP_CONCAT函数实现聚合乘法。

先创建一张演示样例表：

CREATE TABLE `tb_seq` (
  `num` int(10) NOT NULL,
  `seq_type` enum('yellow','green','red') NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

插入演示样例数据：

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 < 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 ;

好了。我们利用我创建的函数以及MYSQL自带的GROUP_CONCAT聚合函数就能够非常方便的实现乘法了。

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)