What are the different mill types?
select distinct type from mill
Order by the number of mills of each type in decreasing order.
select type from mill group by type order by count(*) desc
What is the most common mill type?
select type from mill group by type order by count(*) desc limit 1
How many mills of that type are there?
select type, count(*) from mill group by type order by count(*) desc limit 1