Comment on the following queries

SQL$-1$: SELECT $*$ FROM invoices i INNER JOIN cities c ON i$.$service$\_$city$\_$code $=$ c$.$city$\_$code;

SQL$-2$: SELECT $*$ FROM invoices i JOIN cities c ON i$.$service$\_$city$\_$code $=$ c$.$city$\_$code;

SQL$-3$: SELECT $*$ FROM invoices i$,$ cities c WHERE i$.$service$\_$city$\_$code $=$ c$.$city$\_$code;