Re: Unnexpected nonaggregated field error
> I don't understand. There is no reason for the fields in Pages,
> those starting with PT_ to be aggregated since there is only one
> matching record in Pages for each record obtained from Census1911.
All right, your version is after 5.7.5, which is the version where MySQL learned how to detect functional dependencies in such contexts. If, revising your query to say ...
Group_Concat(PT_Population), Group_Concat(PT_Transcriber), Group_Concat(PT_Proofreader)
just gives lists of length one, you've found a case where MySQL's new functional dependency detection fails, and it might be worth a bug report. If that's the case, the workaround would be to modify the query to say ...
Any_Value(PT_Population), Any_Value(PT_Transcriber), Any_Value(PT_Proofreader)