probleme jointure entre table

public static final String table_facture_f__Name = "facture_f";
static final String[] TABLE_fature_f = 
		{ 
		"n_facture_f varchar(20) NOT NULL" , 
		"date_ff date NOT NULL" ,
		"id_f int not null " ,
		"mt_ht_ff double precision not null ",
		"mt_ttc_ff double precision not null " ,
		"mp double precision ",
		"rp double precision ",
		"PRIMARY KEY (n_facture_f, id_f)" , 
		"CONSTRAINT FKPRq FOREIGN KEY (id_f) REFERENCES fournisseur (id_f) ",
        };
	
public static final String table_contenu_ff_Name = "contenu_ff";
static final String[] TABLE_contenu_ff = 
		{ 
		"n_commande_f int(6) zerofill  NOT NULL" , 
		"n_facture_f varchar(20) NOT NULL" ,
		"id_f int not null",
		"CONSTRAINT qz4 FOREIGN KEY (id_f) REFERENCES fournisseur (id_f) " ,
		"constraint a1e PRIMARY KEY (n_commande_f)",
		"CONSTRAINT a2v FOREIGN KEY (n_commande_f) REFERENCES commande_f (n_commande_f) ", 
		"CONSTRAINT a3xw FOREIGN KEY (n_facture_f) REFERENCES facture_f (n_facture_f) "
        };	

String req2 = "select contenu_ff.n_commande_f, date_cf ,t_ht,t_ttc, livraison  from commande_f, contenu_ff where contenu_ff.id_f = commande_f.id_f  and contenu_ff.n_facture_f = '" + num +"'and contenu_ff.id_f = '"+ obj+"' ";
rs = st.executeQuery(req2);

public static final String table_commande_f_Name = "commande_f";
static final String[] TABLE_commande_f = 
		{ 
		"n_commande_f int(6) zerofill  AUTO_INCREMENT NOT NULL" , 
		"id_f int not null ",
		"date_cf date not null" ,
		"t_ht double precision not null",
		"t_ttc double precision not null ", 
		"livraison varchar(10) not null",
		"facturation varchar(10) not null",
		"PRIMARY KEY (n_commande_f)",
		"CONSTRAINT FK_FCT_PRS FOREIGN KEY (id_f) REFERENCES fournisseur (id_f) " 
        };	

String req2 = "SELECT cont.n_commande_f, comd.date_cf ,comd.t_ht,t_ttc, comd.livraison";
req2 += "FROM commande_f AS comd";
req2 += "INNER JOIN contenu_ff AS cont";
req2 += "ON cont.n_facture_f = '" + num +"'";
req2 += "AND comd.id_f = commande_f.id_f";
req2 += "AND comd.id_f = '"+ obj+"'";
req2 += ";";