Which allergy has the most number of students affected?
select allergy from has_allergy group by allergy order by count ( * ) desc limit 1
How many students has tree pollen affected?
select count ( stuid ) from has_allergy where allergy = 'Tree Pollen'
Which allergy has the least number of students affected?
select allergy from has_allergy group by allergy order by count ( * ) asc limit 1