MySQL : isn't in GROUP BY

Question

The site produces results, but with SELECT COUNT and SELECT query with GROUP BY having two different result counts. This is likely due to the error that is displaying in phpmyadmin but not on the site.

The Queries:

SELECT count(DISTINCT `name`) as `numrows` FROM `users` WHERE `verified` = '1'

SELECT `name`, `type`, `language`, `code` FROM `users` WHERE `verified` = '1' GROUP BY `name` ORDER BY `count` DESC LIMIT 0, 25

PhpMyAdmin provides the following error:

1055 - 'main.users.type' isn't in GROUP BY

When reading MySQL docs, I'm still unclear what it is I have to fix. I can't seem to grasp this.

Answer 1

You need to have a full group by:

SELECT `name`, `type`, `language`, `code` 
FROM `users` 
WHERE `verified` = '1' 
GROUP BY `name`, `type`, `language`, `code` 
ORDER BY `count` DESC LIMIT 0, 25

SQL92 requires that all columns (except aggregates) in the select clause is part of the group by clause. SQL99 loosens this restriction a bit and states that all columns in the select clause must be functionally dependent of the group by clause. MySQL by default allows for partial group by and this may produce non-deterministic answers, example:

create table t (x int, y int);
insert into t (x,y) values (1,1),(1,2),(1,3);
select x,y from t group by x;
+------+------+
| x    | y    |
+------+------+
|    1 |    1 |
+------+------+

I.e. a random y is select for the group x. One can prevent this behavior by setting @@sql_mode:

set @@sql_mode='ONLY_FULL_GROUP_BY';
select x,y from t group by x; 
ERROR 1055 (42000): 'test.t.y' isn't in GROUP BY