Show the location codes.
select location_code from document_locations
For each of them, what is the count of associated documents?
select location_code, count(*) from document_locations group by location_code
Sort codes in descending order by these counts.
select location_code from document_locations group by location_code order by count(*) desc
Which has the most?
select location_code from document_locations group by location_code order by count(*) desc limit 1