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I'm currently trying to make a self referencing table in MySQL, however it seems I can't make a foreign key on the table itself, I'm getting an MySQL error: Error Code: 1005. Can't create table 'biological classification' (errno: 150) This is my code: # Table creation DROP TABLE IF EXISTS `biological classification`; CREATE TABLE `biological classification` ( `idBC` int(10) unsigned NOT NULL AUTO_INCREMENT, `idParent` int(11) DEFAULT NULL, `type` varchar(45) DEFAULT NULL, `value` varchar(45) DEFAULT NULL, PRIMARY KEY (`idBC`), UNIQUE KEY `idnew_table_UNIQUE` (`idBC`), CONSTRAINT `SelfKey` FOREIGN KEY (`idParent`) REFERENCES `biological classification` (`idBC`) ON DELETE NO ACTION ON UPDATE NO ACTION ) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=latin1 COMMENT='A table that contains the Biological Classification of anima'; # insert into table INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (1,NULL,'Class','Mammalia'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (2,1,'Genus','Giraffa'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (3,2,'Species','Giraffa camelopardalis'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (5,1,'Genus','Panthera'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (6,1,'Genus','Loxodonta'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (7,5,'Species','Panthera leo'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (8,5,'Species','Panthera tigris'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (9,6,'Species','Loxodonta africana'); INSERT INTO `biological classification` (`idBC`,`idParent`,`type`,`value`) VALUES (10,1,'Class','Marsupialia'); INSERT INTO `biological classification` (`idBC`,`idP

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