GROUP BY foreign key or primary key?

Question

I have two tables :

user
=======
id
name
class

marks
=======
id
user_id
sub_id
mark

user table contains the details of user (student) marks table contains the marks of a student in different subjects with subject id

I want to fetch name, class and total marks from these tables.

I have two queries :

1. SELECT name, class, SUM(mark) AS total FROM user
   LEFT JOIN marks ON marks.user_id = user.id
   GROUP BY marks.user_id
   ///Here in GROUP BY I have used foreign key (marks.user_id)

2. SELECT name, class, SUM(mark) AS total FROM user
   LEFT JOIN marks ON marks.user_id = user.id
   GROUP BY user.id
   ///Here in GROUP BY I have used primary key (user.id)

Both gives me required data. My question is which one should I use?

Is there any rule which says you should use primary key in group by OR foreign key in group by OR something like that ?

Answer 1

Given you have done a left join you should use the 2nd query as marks.user_id could be null