I am creating 2 tables in my database:
DROP TABLE IF EXISTS `med_pharmacy`;
CREATE TABLE IF NOT EXISTS `med_pharmacy` (
`med_pharmacy_id` int(11) NOT NULL AUTO_INCREMENT,
`med_id` int(11) NOT NULL,
`med_barcode` varchar(45) DEFAULT NULL,
`med_received` date DEFAULT NULL,
`med_expiry` date DEFAULT NULL,
`med_tablet` int(11) DEFAULT NULL,
`med_pill` int(11) DEFAULT NULL,
`clinic_id` varchar(45) DEFAULT NULL,
PRIMARY KEY (`med_pharmacy_id`),
KEY `fk_med_pharmacy_medication1_idx` (`med_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1261 DEFAULT CHARSET=utf8mb4;
AND:
DROP TABLE IF EXISTS `medication`;
CREATE TABLE `medication` (
`med_id` int(11) NOT NULL,
`med_name` varchar(75) NOT NULL,
`med_date_added` date DEFAULT NULL,
`clinic_id` varchar(45) DEFAULT NULL,
`med_type` varchar(15) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
And when I run the queries in wamp I got this error:
SQL query:
ALTER TABLE `med_pharmacy`
ADD CONSTRAINT `fk_med_pharmacy_medication1`
FOREIGN KEY (`med_id`)
REFERENCES
`medication` (`med_id`) ON DELETE CASCADE ON UPDATE CASCADE MySQL
said: Documentation
#1822 - Failed to add the foreign key constaint. Missing index for constraint 'fk_med_pharmacy_medication1' in the referenced table 'medication'
The tables already exists but I changed one field.
2 Answers
The column referenced in a foreign key must be indexed. You need to add an index on medication.med_id. In fact, this should probably be the primary key of the table.
ALTER TABLE medication ADD PRIMARY KEY (med_id);
if you are giving
ADD CONSTRAINT `fk_med_pharmacy_medication1`
FOREIGN KEY (`med_id`)
A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
so med_id should have primary key in medication or reference the columns of a UNIQUE constraint
