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

Which allergy is the most common?
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 FETCH FIRST 1 ROWS ONLY