If the MySQL's only_full_group_by mode is turned on it will mean that the strict ANSI SQL rules will be applicable while using GROUP BY clause. With respect to the query, it suggests that if the GROUP BY of the proof_type column used, then you can only select two things:
· your proof_type column, or
· the aggregates of any other column
The "aggregates" of any other columns means use of the aggregate function e.g. MIN(), MAX(), or AVG() with another column.
In the majority of MySQL GROUP BY questions which come to me so I have strict mode turned off and so the query is running with incorrect results. But in your case the query will not run at all that’s why you are thinking about what you actually want to do.