Untitled

 show create table rental;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                                                     |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| rental | CREATE TABLE `rental` (
  `driverid` int(11) NOT NULL DEFAULT '0',
  `carid` int(11) NOT NULL DEFAULT '0',
  `sdate` date NOT NULL DEFAULT '0000-00-00',
  `edate` date DEFAULT NULL,
  `rdate` date DEFAULT NULL,
  `cost` int(11) DEFAULT NULL,
  PRIMARY KEY (`driverid`,`carid`,`sdate`),
  KEY `fk_1_cust` (`carid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table rental add constraint fk_2_cust foreign key (driverid) references driverid;
Query OK, 7 rows affected (0.15 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> show create table rental;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                                                     |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| rental | CREATE TABLE `rental` (
  `driverid` int(11) NOT NULL DEFAULT '0',
  `carid` int(11) NOT NULL DEFAULT '0',
  `sdate` date NOT NULL DEFAULT '0000-00-00',
  `edate` date DEFAULT NULL,
  `rdate` date DEFAULT NULL,
  `cost` int(11) DEFAULT NULL,
  PRIMARY KEY (`driverid`,`carid`,`sdate`),
  KEY `fk_1_cust` (`carid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)