MySQL 8 does not detect functional dependencies in select distinct queries - mysql

Question

I create a table using this query:
create table a (
`id` int not null auto_increment,
b varchar(10),
primary key (`id`)
);
Executing
select distinct `id` from a order by `b`;
results in this error:
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
But if I change the query to
select `id` from a group by `id` order by `b`;
which is logically equivalent, it succeeds.
I'm using the official Docker image for MySQL and mysql --version displays
mysql Ver 8.0.12 for Linux on x86_64 (MySQL Community Server - GPL)
It seems that MySQL does not still detect functional dependencies in select distinct queries. Am I right? Are MySQL's developers going to fix this?

Answer 1

The opposite behaviour was actually reported as a bug and 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.
Also, the documentation explicitly states that this is the intended behaviour:
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
without mentioning functional dependencies. In contrast to group by, the relevant error message does not reference functional dependencies either.
While the optional feature T301 Functional dependencies in the sql standard does modify the conformity rules for group by (and others), it doesn't change any restriction on order by plus distinct, which means it is still forbidden.

Answer 2

This comes from the introduction of a new SQL MODE from MySQL 5.7 : ONLY_FULL_GROUP_BY
The goal of this mode was to make MySQL behave as the SQL standard on GROUP BY queries.
This mode is activated on your database. You can either disable it, or adapt your queries to respect standards, which is probably the best thing to do.
This will throw your error:
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`;
If you remove the ONLY_FULL_GROUP_BY mode, you get your results :
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`;
You can see it live on Rextester