### Database Schema:
CREATE TABLE connected (
	atom_id	TEXT	NOT NULL	PRIMARY KEY,
	atom_id2	TEXT	NOT NULL	PRIMARY KEY,
	bond_id	TEXT	NULL	DEFAULT NULL,
	foreign key (atom_id2) references atom (atom_id),
	foreign key (atom_id) references atom (atom_id),
	foreign key (bond_id) references bond (bond_id)
);

CREATE TABLE atom (
	atom_id	TEXT	NOT NULL	PRIMARY KEY,
	molecule_id	TEXT	NULL	DEFAULT NULL,
	element	TEXT	NULL	DEFAULT NULL,
	foreign key (molecule_id) references molecule (molecule_id)
);

CREATE TABLE bond (
	bond_id	TEXT	NOT NULL	PRIMARY KEY,
	molecule_id	TEXT	NULL	DEFAULT NULL,
	bond_type	TEXT	NULL	DEFAULT NULL,
	foreign key (molecule_id) references molecule (molecule_id)
);

CREATE TABLE molecule (
	molecule_id	TEXT	NOT NULL	PRIMARY KEY,
	label	TEXT	NULL	DEFAULT NULL
);

### User's Query:
On average how many carcinogenic molecules are single bonded?

### Extra Knowledge:
carcinogenic molecules refers to label = '+'; single-bonded refers to bond_type = '-'; average = DIVIDE(SUM(bond_type = '-'), COUNT(atom_id))
SELECT AVG(single_bond_count) FROM (SELECT T3.molecule_id, COUNT(T1.bond_type) AS single_bond_count FROM bond AS T1  INNER JOIN atom AS T2 ON T1.molecule_id = T2.molecule_id INNER JOIN molecule AS T3 ON T3.molecule_id = T2.molecule_id WHERE T1.bond_type = '-' AND T3.label = '+' GROUP BY T3.molecule_id) AS subquery