需求

找出最近练习过的句子id，不能重复（练习保存在practice表中，句子保存在pth_sentences表中，一个pth_sentence纪录有多条practices纪录）。现在的情况是如果一个句子练习多次，会得到重复的pth_sentence_id，这样前端在列出来的时候呢，句子是重复的，我只想让句子显示一次，既pth_sentence_id只出现一次。

Practices的表结构是这样的：

CREATE TABLE `practices` ( `id` bigint(20) NOT NULL AUTO_INCREMENT, `user_id` int(11) DEFAULT NULL, `pth_sentence_id` int(11) DEFAULT NULL, `local_sound` varchar(255) DEFAULT NULL, `comment` varchar(255) DEFAULT NULL, `created_at` datetime(6) NOT NULL, `updated_at` datetime(6) NOT NULL, `soundfile` json DEFAULT NULL, `del` tinyint(1) DEFAULT '0', PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=215 DEFAULT CHARSET=utf8mb4;

一步步写出满足需求的SQL语句好喽，以前老师就是这么教的。

最近句子练习纪录5条

so easy. 问题就是句子id有重复，需要去重。

加入group by

select * from practicesorder by created_at descgroup by pth_sentence_idlimit 5

出错了

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'group by pth_sentence_id limit 10' at line 3

为啥呢？order by 需要在group by 的后面执行！并且要用where语句的话需要放在group by之前！

调整下顺序：

select * from practicesgroup by pth_sentence_idorder by created_at desclimit 5

还是出错，请原谅我好久没写SQL了：

1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'beginner.practices.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

为啥呢？因为id没有写在group by 语句中， group by 是要与聚合函数结合起来使用的。

好，我不写id了，改成这样呢？

select pth_sentence_id from practicesgroup by pth_sentence_idorder by created_at desclimit 5

继续出错。

1055 - Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'beginner.practices.created_at' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

这回是group by中的created_at呢，说没有在group_by语句中。好，加进去。

倒是没错了，但结果不是我想要的，因为这连个id都没有，而且数据有重复。