CREATE TABLE allergy_type (allergy TEXT, allergytype TEXT)
CREATE TABLE has_allergy (stuid DECIMAL, allergy TEXT)
CREATE TABLE student (stuid DECIMAL, lname TEXT, fname TEXT, age DECIMAL, sex TEXT, major DECIMAL, advisor DECIMAL, city_code TEXT)

Which allergy has most number of students affected?
SELECT allergy FROM has_allergy GROUP BY allergy ORDER BY COUNT(*) DESC LIMIT 1
SELECT "allergy" FROM "has_allergy" GROUP BY "allergy" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1