如何在严格模式下正确使用分组方式？

Question

我有这样的消息表：

CREATE TABLE `message` (
    `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
    `from_id` bigint(20) unsigned NOT NULL,
    `to_id` bigint(20) unsigned NOT NULL,
    `body` text COLLATE utf8mb4_unicode_ci NOT NULL,
    `status` tinyint(4) NOT NULL,
    `created_at` timestamp NULL DEFAULT NULL,  
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

并尝试选择每一个最后一个从userX到userY的消息，但是mysql总是说我有未聚合的列（没有only_full_group_by工作正常）。如何在严格模式下选择此项？无效的查询，例如：

select
     t1.created_at,
     t1.from_id,
     t1.to_id,
     t1.body,
     t1.status,
     ( select created_at from test1.message where from_id = t1.from_id order by created_at desc limit 1 ) as last_timestamp

from test1.message as t1
group by t1.from_id 
having t1.created_at = last_timestamp

Answer 1

不聚合-而是过滤。

一个选项使用相关子查询，该子查询计算每个用户的最大created_at：

 select m.*
 from message m
 where m.created_at = (
      select max(m1.created_at) from message m1 where m1.from_id = m.from_id
)

您还可以使用反left join模式：

select m.*
from message m
left join message m1 on m1.from_id = m.from_id and m1.createt_at > m.created_at
where m1.id is null

此短语为：获取没有其他记录的记录，这些记录具有相同的from_id和更大的created_at。

Answer 2

您甚至不需要在这里使用GROUP BY，没有它，您当前的查询实际上应该是有效的：

SELECT
    t1.created_at,
    t1.from_id,
    t1.to_id,
    t1.body,
    t1.status,
    (SELECT t2.created_at FROM test1.message t2
     WHERE t2.from_id = t1.from_id
     ORDER BY t2.created_at DESC LIMIT 1) AS last_timestamp
FROM test1.message AS t1
HAVING t1.created_at = last_timestamp;

MySQL重载了HAVING运算符，使其可以代替WHERE子句使用，其附加功能是实际上可以在其中使用在SELECT子句中定义的别名。