mysql> create table n (
    -> i int not null primary key
    -> );
Query OK,  0  rows affected ( 0 . 13  sec)

mysql> create table m (
    -> j int not null primary key
    -> );
Query OK,  0  rows affected ( 0 . 11  sec)

mysql> insert into n values ( 1 );
Query OK,  1  row affected ( 0 . 03  sec)

mysql> insert into n values ( 2 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> insert into n values ( 3 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> insert into m values ( 1 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> insert into m values ( 2 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> insert into m values ( 3 );
Query OK,  1  row affected ( 0 . 02  sec)

mysql> create table mn (
    -> i int not null,
    -> j int not null,
    -> foreign key (i) references n (i),
    -> foreign key (j) references m (j)
    -> );
Query OK,  0  rows affected ( 0 . 30  sec)

mysql> insert into mn (i, j) values ( 1 ,  1 );
Query OK,  1  row affected ( 0 . 03  sec)

mysql> insert into mn (i, j) values ( 1 ,  2 );
Query OK,  1  row affected ( 0 . 03  sec)

mysql> update mn set j= 3  where i= 1  and j= 2 ;
Query OK,  1  row affected ( 0 . 02  sec)
Rows matched:  1   Changed:  1   Warnings:  0