CREATE TABLE Has_allergy (allergy VARCHAR(255)); CREATE TABLE Allergy_type (allergytype VARCHAR(255), allergy VARCHAR(255))
Show all allergy type with number of students affected.
    
SQL: SELECT T2.allergytype, COUNT(*) FROM Has_allergy AS T1 JOIN Allergy_type AS T2 ON T1.allergy = T2.allergy GROUP BY T2.allergytype