Cannot add foreign key constraint in mysql-workbench - mysql

Question

The following sql script causes the Error Code: 1215. Cannot add foreign key constraint in MySQL Workbench
DROP TABLE IF EXISTS `bookings`;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `bookings` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`van_id` int(11) NOT NULL,
`driver_id` int(11) NOT NULL,
`route_id` int(11) NOT NULL,
`registered_seats` int(11) NOT NULL,
`departure_time` varchar(11) NOT NULL,
`arival_time` varchar(11) NOT NULL,
`departure_date` varchar(11) NOT NULL,
`total_cost` int(11) NOT NULL,
`expected_price` int(11) NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (`van_id`) REFERENCES `van` (`id`) ON DELETE SET NULL,
FOREIGN KEY (`driver_id`) REFERENCES `driver` (`id`) ON DELETE SET NULL,
FOREIGN KEY (`route_id`) REFERENCES `route` (`id`) ON DELETE SET NULL
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;
For reference, below are the scripts of the related tables, all of these work fine.
For table van,
DROP TABLE IF EXISTS `van`;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `van` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`vehicle_registration` varchar(45) NOT NULL UNIQUE,
`vehicle_type` varchar(45) NOT NULL,
`total_seats` int(11) NOT NULL,
`category` varchar(45) NOT NULL,
`flagged` int(2) DEFAULT 0 NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;
For table Driver,
DROP TABLE IF EXISTS `driver`;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `driver` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`first_name` varchar(45) NOT NULL ,
`last_name` varchar(45) DEFAULT NULL,
`cnic` varchar(45) NOT NULL UNIQUE,
`license_number` varchar(11) NOT NULL UNIQUE,
`phonenumber` int(11) NOT NULL unique,
`picture` varchar(15) NOT NULL unique,
`flagged` int(2) DEFAULT 0,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;
For table route,
DROP TABLE IF EXISTS `route`;
/*!40101 SET #saved_cs_client = ##character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `route` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`source` varchar(45) NOT NULL,
`destination` varchar(45) NOT NULL,
`exp_t_time` varchar(45) NOT NULL,
`eco_fare` int(11) NOT NULL,
`ac_fare` int(11) NOT NULL,
`state` varchar(45) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = #saved_cs_client */;

Answer 1

I was trying to SET NULL an attribute (in the foreign key constraint) that is supposed to be NOT NULL.
I made the following changes in the bookings mysql script.
Before:
FOREIGN KEY (`van_id`) REFERENCES `van` (`id`) ON DELETE SET NULL,
FOREIGN KEY (`driver_id`) REFERENCES `driver` (`id`) ON DELETE SET NULL,
FOREIGN KEY (`route_id`) REFERENCES `route` (`id`) ON DELETE SET NULL
After:
FOREIGN KEY (`van_id`) REFERENCES `van` (`id`) ON DELETE CASCADE,
FOREIGN KEY (`driver_id`) REFERENCES `driver` (`id`) ON DELETE CASCADE,
FOREIGN KEY (`route_id`) REFERENCES `route` (`id`) ON DELETE CASCADE