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mysql> create table ug1 (gid int not null, uid int not null, primary key (uid, gid));
Query OK, 0 rows affected (0.01 sec)

mysql> create table ug2 (gid int not null, uid int not null, foreign key (uid) references ug1(uid) on update cascade on delete restrict);
Query OK, 0 rows affected (0.01 sec)
...
mysql> select * from ug2;
+-----+-----+
| gid | uid |
+-----+-----+
|   1 |   1 | 
|   2 |   2 | 
|   3 |   3 | 
|   4 |   4 | 
|   5 |   5 | 
|   6 |   6 | 
+-----+-----+
6 rows in set (0.00 sec)

mysql> delete from ug2 where uid = 1;
Query OK, 1 row affected (0.00 sec)

mysql> select * from ug2;
+-----+-----+
| gid | uid |
+-----+-----+
|   2 |   2 | 
|   3 |   3 | 
|   4 |   4 | 
|   5 |   5 | 
|   6 |   6 | 
+-----+-----+
5 rows in set (0.00 sec)

mysql> insert ug1 values (6,6)
    -> insert ug1 values (6,6);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'insert ug1 values (6,6)' at line 2
mysql> insert ug1 values (6,6);
Query OK, 1 row affected (0.00 sec)

mysql> insert ug1 values (4,4);
Query OK, 1 row affected (0.00 sec)

mysql> insert ug1 values (4,4);
ERROR 1062 (23000): Duplicate entry '4-4' for key 1
mysql> insert ug1 values (3,3);
Query OK, 1 row affected (0.00 sec)

mysql> delete from ug2 where uid = 3;
Query OK, 1 row affected (0.00 sec)

mysql> select * from ug2;
+-----+-----+
| gid | uid |
+-----+-----+
|   2 |   2 | 
|   4 |   4 | 
|   5 |   5 | 
|   6 |   6 | 
+-----+-----+
4 rows in set (0.00 sec)

mysql> select * from ug1;
+-----+-----+
| gid | uid |
+-----+-----+
|   3 |   3 | 
|   4 |   4 | 
|   6 |   6 | 
+-----+-----+
3 rows in set (0.00 sec)