Database: toxicology
Question: What is the percentage of carbon in double-bond molecules?
SQL: SELECT CAST(COUNT(DISTINCT CASE WHEN T1.element = 'c' THEN T1.atom_id ELSE NULL END) AS REAL) * 100 / COUNT(DISTINCT T1.atom_id) FROM atom AS T1 INNER JOIN bond AS T2 ON T1.molecule_id = T2.molecule_id WHERE T2.bond_type = '='