GROUP_CONCAT()按文本字段分组时的行数

Question

DROP TABLE IF EXISTS `table`;
CREATE TABLE `table` (
  `id` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
  `text` text COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO `table` VALUES ('1', 'Unpacked reserved sir offering bed judgment may and quitting speaking. Is do be improved raptures offering required in replying raillery. Stairs ladies friend by in mutual an no. Mr hence chief he cause. Whole no doors on hoped. Mile tell if help they ye full name. \r\n\r\nLadyship it daughter securing procured or am moreover mr. Put sir she exercise vicinity cheerful wondered. Continual say suspicion provision you neglected sir curiosity unwilling. Simplicity end themselves increasing led day sympathize yet. General windows effects not are drawing man garrets. Common indeed garden you his ladies out yet. Preference imprudence contrasted to remarkably in on. Taken now you him trees tears any. Her object giving end sister except oppose. \r\n\r\nWas justice improve age article between. No projection as up preference reasonably delightful celebrated. Preserved and abilities assurance tolerably breakfast use saw. And painted letters forming far village elderly compact. Her rest west each spot his and you knew. Estate gay wooded depart six far her. Of we be have it lose gate bred. Do separate removing or expenses in. Had covered but evident chapter matters anxious.');
INSERT INTO `table` VALUES ('2', 'Unpacked reserved sir offering bed judgment may and quitting speaking. Is do be improved raptures offering required in replying raillery. Stairs ladies friend by in mutual an no. Mr hence chief he cause. Whole no doors on hoped. Mile tell if help they ye full name. \r\n\r\nLadyship it daughter securing procured or am moreover mr. Put sir she exercise vicinity cheerful wondered. Continual say suspicion provision you neglected sir curiosity unwilling. Simplicity end themselves increasing led day sympathize yet. General windows effects not are drawing man garrets. Common indeed garden you his ladies out yet. Preference imprudence contrasted to remarkably in on. Taken now you him trees tears any. Her object giving end sister except oppose. \r\n\r\nWas justice improve age article between. No projection as up preference reasonably delightful celebrated. Preserved and abilities assurance tolerably breakfast use saw. And painted letters forming far village elderly compact. Her rest west each spot his and you knew. Estate gay wooded depart six far her. Of we be have it lose gate bred. Do separate removing or expenses in. Had covered but evident chapter matters anxious');

在GROUP BY不使用GROUP_CONCAT()结果集的情况下运行查询时是预期的(显示两行,每个变量一行text):

SELECT
    `text`
FROM
    `table`
GROUP BY
    `text`;

+-----------------------------------+
| text                              |
+-----------------------------------+
| Unpacked reserved sir offering... |
| Unpacked reserved sir offering... |
+-----------------------------------+
2 rows in set (0.02 sec)

但是,当使用GROUP_CONCAT()结果集运行相同的查询时不符合预期(显示一行具有两个id字段的串联字符串):

SELECT
    GROUP_CONCAT(`id` SEPARATOR ', ') AS ids
FROM
    `table`
GROUP BY
    `text`;

+------+
| ids  |
+------+
| 1, 2 |
+------+
1 row in set (0.00 sec)

我的问题:

为什么使用GROUP_CONCAT()会影响返回的行数？

我最初的假设是它GROUP_CONCAT_MAX_LEN与它有关(我的设置为1024)但肯定只会影响GROUP_CONCAT(),而不是GROUP BY(也正如你所注意到的,我GROUP_CONCAT()在id场上使用,而不是text场,以及那个结果甚至没有接近超过GROUP_CONCAT_MAX_LEN).

Answer 1

您必须根据需要将max_sort_length更改为更高的会话数或全局数.默认情况下,其值为1024个字节,并且您的字符串包含1170个字节的数据.通过增加大小,它将为GROUP_CONCAT提供两行.

检查此链接max_sort_length

SELECT `text` FROM `table` GROUP BY `text`;

SET SESSION max_sort_length = 2000;
SELECT GROUP_CONCAT(`id` SEPARATOR ', ') AS ids FROM `table` GROUP BY `text`;

检查SQL FIDDLE DEMO

编辑: BLOB和TEXT值无法在GROUP BY,ORDER BY或DISTINCT中可靠地使用.在这些情况下比较BLOB值时,仅使用第一个max_sort_length字节.max_sort_length的默认值为1024,可以在服务器启动时或运行时更改.