### 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:
What is the composition of element chlorine in percentage among the single bond molecules?

### Extra Knowledge:
element chlorine refers to element = 'cl'; single bond refers to bond_type = '-'; percentage = DIVIDE(SUM(element = 'cl'), COUNT(atom_id)) as percent where bond_type = '-'
SELECT CAST(COUNT(CASE WHEN T.element = 'cl' THEN T.atom_id ELSE NULL END) AS REAL) * 100 / COUNT(T.atom_id) FROM ( SELECT T1.atom_id, T1.element FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id INNER JOIN bond AS T3 ON T2.molecule_id = T3.molecule_id WHERE T3.bond_type = '-' ) AS T