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');

Когда работаетGROUP BY запрос без использованияGROUP_CONCAT() набор результатов соответствует ожиданиям (показаны две строки, по одной для каждого вариантаtext):

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

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

Однако при выполнении того же запроса сGROUP_CONCAT() набор результатов не такой, как ожидалось (показывает одну строку с объединенной строкой из двухid поля):

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

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

Мой вопрос:

Зачем бы использоватьGROUP_CONCAT() влияет на количество возвращаемых строк?

Мое первоначальное предположение было, чтоGROUP_CONCAT_MAX_LEN имеет какое-то отношение к этому (у меня установлено 1024), но, конечно, это влияет толькоGROUP_CONCAT()неGROUP BY (также, как вы можете заметить, я используюGROUP_CONCAT() наid поле, а неtext поле, и результат этого даже не приближается к превышениюGROUP_CONCAT_MAX_LEN).