Can't fetch a field with GROUP BY clause - mysql

Question

I'm trying to create a simple query that will find a person with highest average marks and display some basic information about them. It's retrieving the proper record, but I can't make MySQL display students.classId field. The error I'm getting in LibreOffice Base is
Not in aggregate function or group by clause.
Query with error:
SELECT CONCAT(`students`.`surname`, CONCAT(' ', `students`.`name`)) AS `Student`,
AVG(CAST(`marks`.`mark` AS DECIMAL (10, 2))) AS `Average`,
`students`.`classId`
FROM `students`, `marks`, `subjects`
WHERE `marks`.`subjectId` = `subjects`.`subjectId`
AND `students`.`studentId` = `marks`.`markId`
GROUP BY `students`.`surname`, `students`.`name`
ORDER BY `Average` DESC LIMIT 1;
Query without error:
SELECT CONCAT(`students`.`surname`, CONCAT(' ', `students`.`name`)) AS `Student`,
AVG(CAST(`marks`.`mark` AS DECIMAL (10, 2))) AS `Average`
FROM `students`, `marks`, `subjects`
WHERE `marks`.`subjectId` = `subjects`.`subjectId`
AND `students`.`studentId` = `marks`.`markId`
GROUP BY `students`.`surname`, `students`.`name`
ORDER BY `Average` DESC LIMIT 1;
I'm not really experienced with SQL, but I think that posting table definitions isn't necessary in this case. If I am wrong, please leave a note in the comments, I'll update the question as soon as possible.
Please note that it is not a homework.

Answer 1

The problematic item is this:
`students`.`classId`
Since the GROUP BY query produces a single row for one or more rows of the joined tables, that single row may correspond to more than one students.classId value.
That is what SQL is asking you to fix: it wants to know which of potentially many items of students.classId you want it to return. The two choices are adding an aggregate function, say
MIN(`students`.`classId`) AS StudentClassId
or using students.classId in the GROUP BY clause:
GROUP BY `students`.`surname`, `students`.`name`, `students`.`classId`
Note that if you go with the later choice, the aggregation would be per student / class pair, not per student.