Calculate the percentage of carcinogenic molecules which contain the Chlorine element.
label = '+' mean molecules are carcinogenic; percentage = DIVIDE(SUM(label = '+' and element = 'cl'), COUNT(molecule_id)) as percentage
SELECT COUNT(CASE WHEN T2.label = '+' AND T1.element = 'cl' THEN T2.molecule_id ELSE NULL END) * 100 / COUNT(T2.molecule_id) FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id