mysql> CREATE TABLE t1 (id int, index(id)) ENGINE=INNODB;
Query OK, 0 rows affected (0.08 sec)

mysql> CREATE TABLE t2 (id int, index(id), FOREIGN KEY (id) REFERENCES t1 (id)) ENGINE=InnoDB;
Query OK, 0 rows affected (0.09 sec)

mysql> INSERT INTO t1 VALUES (1), (1);
Query OK, 2 rows affected (0.05 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> INSERT INTO t2 VALUES (1), (1);
Query OK, 2 rows affected (0.03 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t1; SELECT * FROM t2;
+------+
| id   |
+------+
|    1 |
|    1 |
+------+
2 rows in set (0.00 sec)

+------+
| id   |
+------+
|    1 |
|    1 |
+------+
2 rows in set (0.00 sec)

mysql> DELETE FROM t1 LIMIT 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test/t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`id`) REFERENCES `t1` (`id`))