MySQL GROUP_CONCAT 砍掉一半的数据（有时）

Posted 2023-02-24

【中文标题】MySQL GROUP_CONCAT 砍掉一半的数据（有时）

【英文标题】：MySQL GROUP_CONCAT chopping off half the data (sometimes)

【发布时间】：2021-12-21 05:50:00

【问题描述】：

我似乎有一个非常奇怪的问题...以下查询截断了GROUP_CONCAT 的一些结果

select
    booking_id,
    group_concat(booking_resource_info_grouped_bla)
from (
    SELECT
            b.id AS booking_id,
            GROUP_CONCAT(DISTINCT booking_resource_info SEPARATOR 'hr') AS `booking_resource_info_grouped_bla`
    FROM
    (
        SELECT 492185 AS id, '25ft Climbing 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, '25ft Climbing 2' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, '3 G Swing' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Archery 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Body Boarding' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Canadian Canoes' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Catering 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Crate Stack 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Kayak 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Kayak 2' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Orange Lodge' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Lake' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Low Ropes' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Marquee 1 - Site 3' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Parker Lodge' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Raft Building 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Raft Building 2' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Raft Building 3' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'School Package 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Apple Lodge' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Team Building 1' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Tomahawks' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Banana Lodge' as booking_resource_info
        UNION ALL
        SELECT 492185 AS id, 'Wobble Pole' as booking_resource_info
    ) b
    GROUP BY b.id
    UNION ALL
    SELECT
        123 AS booking_id,
        '' AS `booking_resource_info_grouped_bla`
) test
GROUP BY booking_id
;

这会产生：

25ft Climbing 1hr25ft Climbing 2hr3 G SwinghrApple LodgehrArchery 1hrBanana Lodgehr身体 登机hr加拿大独木舟hr餐饮 1hr板条箱堆栈 1hr皮划艇 1hr皮划艇 2hrKiwi Lodgehr湖hr低 绳索hrMarquee 1 - Site 3hrOrange Lodgehr木筏建筑 1hrRaft Building 2hrRaft Bui

如你所见，尾部Raft Bui不完整。

另一方面，我已经让这个查询正常工作。我让它在另一台服务器上工作，所以我重新启动了其中一个，然后那个也开始工作了。

我在 mysql 5.6 和 5.7 上都进行了广泛的尝试——这两个版本都产生了预期和未预期的结果。 我在 db-fiddle.com 上尝试了 MySQL 8.0，这是唯一产生预期结果的版本。

是否有我缺少的设置/内存限制/已知问题？

值得注意的是，当查询的 UNION ALL 部分被删除时，此查询始终有效，因此其中存在一个虚拟联合，因此问题是可重新创建的。

【问题讨论】：

select @@session.group_concat_max_len 在您遇到“问题”的服务器上返回什么？

【参考方案1】：

SET SESSION group_concat_max_len = 1000000;

这将解决您的问题。 默认组 concat 的最大长度为 1024

更多信息：https://www.namasteui.com/mysql-group_concat-maximum-length/

【讨论】：

是的，这似乎有效，谢谢！

看起来很奇怪，它总是在没有最终联合的情况下工作。也许这会加倍长度或类似的奇怪东西。

UNION without ALL 将删除重复项，因此 (foo),(foo),(bar)（3 行）将变为 (foo),(bar)（2 行）