alter table invoice add foreign key (parts_id) references parts (id);
alter table ar add foreign key (customer_id) references customer (id);
alter table ap add foreign key (vendor_id) references vendor (id);
alter table invoice add foreign key (parts_id) references parts (id);
alter table ar add foreign key (customer_id) references customer (id);
alter table ap add foreign key (vendor_id) references vendor (id);