Show all allergies with number of students affected.
SELECT Allergy ,  count(*) FROM Has_allergy GROUP BY Allergy
SELECT Allergy ,  count(*) FROM Has_allergy GROUP BY Allergy
[{'Allergy': 'Anchovies', 'count(*)': 3}, {'Allergy': 'Cat', 'count(*)': 4}, {'Allergy': 'Dog', 'count(*)': 3}, {'Allergy': 'Eggs', 'count(*)': 3}, {'Allergy': 'Grass Pollen', 'count(*)': 4}, {'Allergy': 'Milk', 'count(*)': 3}, {'Allergy': 'Nuts', 'count(*)': 11}, {'Allergy': 'Ragweed', 'count(*)': 6}, {'Allergy': 'Rodent', 'count(*)': 1}, {'Allergy': 'Shellfish', 'count(*)': 4}, {'Allergy': 'Soy', 'count(*)': 4}, {'Allergy': 'Tree Pollen', 'count(*)': 13}]