MySQL 8 不检测 select 不同查询中的函数依赖

Question

我使用此查询创建了一个 table：

create table a (
  `id` int not null auto_increment,
  b varchar(10),
  primary key (`id`)
);

正在执行

select distinct `id` from a order by `b`;

导致此错误：

ERROR 3065 (HY000): Expression #1 of ORDER BY clause is not in SELECT list, references column 'portal.a.b' which is not in SELECT list; this is incompatible with DISTINCT

但是如果我将查询更改为

select `id` from a group by `id` order by `b`;

逻辑上等价，成功

我正在使用 official Docker image for MySQL 和 mysql --version 显示器

mysql Ver 8.0.12 for Linux on x86_64 (MySQL Community Server - GPL)

似乎 MySQL 仍未检测到 select 不同查询中的功能依赖性。我对吗？ MySQL 的开发人员会解决这个问题吗？

Answer 1

这来自 MySQL 5.7 引入的新 SQL 模式：ONLY_FULL_GROUP_BY

此模式的目标是使 MySQL 在 GROUP BY 查询中表现得像 SQL 标准。

此模式已在您的数据库上激活。您可以禁用它，或调整您的查询以遵守标准，这可能是最好的做法。

这将引发您的错误：

SET sql_mode = 'ONLY_FULL_GROUP_BY';

drop table a;
create table a (
  `id` int not null auto_increment,
  b varchar(10),
  primary key (`id`)
);
INSERT INTO a VALUES (NULL, 'aaaa');
INSERT INTO a VALUES (NULL, 'bbbb');

select distinct `id` from a order by `b`;

如果删除 ONLY_FULL_GROUP_BY 模式，您将得到结果：

SET sql_mode = '';

drop table a;
create table a (
  `id` int not null auto_increment,
  b varchar(10),
  primary key (`id`)
);
INSERT INTO a VALUES (NULL, 'aaaa');
INSERT INTO a VALUES (NULL, 'bbbb');

select distinct `id` from a order by `b`;

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Answer 2

相反的行为实际上被报告为错误并且fixed in MySQL 5.7.5:

Several issues related to the ONLY_FULL_GROUP_BY SQL mode were corrected:

With ONLY_FULL_GROUP_BY enabled, some valid queries were rejected if the accessed table was replaced by a view.

Queries of the form SELECT DISTINCT col1 ... ORDER BY col2 qualify as forbidden by SQL2003 (hidden ORDER BY columns combined with DISTINCT), but were not rejected with the ONLY_FULL_GROUP_BY SQL mode enabled.

此外，documentation 明确指出这是预期的行为：

To prevent this problem, a query that has DISTINCT and ORDER BY is rejected as invalid if any ORDER BY expression does not satisfy at least one of these conditions:

The expression is equal to one in the select list

All columns referenced by the expression and belonging to the query's selected tables are elements of the select list

没有提到函数依赖。与group by相反，相关错误消息也没有引用功能依赖项。

虽然 sql 标准中的可选特性 T301 功能依赖性 确实修改了 [=10= 的一致性规则]（和其他人），它不会改变对 order by 加上 distinct 的任何限制，这意味着它仍然是禁止的。

MySQL 8 不检测 select 不同查询中的函数依赖

MySQL 8 does not detect functional dependencies in select distinct queries

mysql

select

distinct

functional-dependencies