As a rule of thumb: When you mix normal columns with aggregate functions in SELECT, you need to use GROUP BY. Do not use GROUP BY when you do not have normal columns and aggregate functions in SELECT.
The thing to put into the GROUP BY, is all from SELECT but the aggregate functions (and possible constants).
As an example if you have a query:
select a, substring(b,3), 'x', max(y)
from yourtable
You need to use GROUP BY. You leave out 'x' as it is a constant and you leave out the aggregate function. The rest goes to the GROUP BY.
select a, substring(b,3), 'x', max(y)
from yourtable
group by a, substring(b,3)
Previous MySQL versions allowed quite liberal use of GROUP BY resulting quite often just bad/incorrect code.
