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Add foreign key

Foreign keys helps prevent admin input errors, producing a more accurate database. Using constraint any attempt to link to a record which does not already exist will fail. 
 
ALTER TABLE article_log ADD FOREIGN KEY (article_id) 
REFERENCES article (id) ON DELETE CASCADE;

Modify foreign key 

 
SHOW CREATE TABLE catg_path;

CREATE TABLE `catg_path` (
  `ancestor` int(10) unsigned NOT NULL,
  `descendant` int(10) unsigned NOT NULL,
  PRIMARY KEY  (`ancestor`,`descendant`),
  KEY `descendant` (`descendant`),
  CONSTRAINT `catg_path_ibfk_1` FOREIGN KEY (`ancestor`) REFERENCES `catg` (`id`),
  CONSTRAINT `catg_path_ibfk_2` FOREIGN KEY (`descendant`) REFERENCES `catg` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
;

ALTER TABLE catg_path DROP FOREIGN KEY catg_path_ibfk_1;

ALTER TABLE catg_path ADD FOREIGN KEY (ancestor) REFERENCES catg (id)
ON DELETE CASCADE;




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