Question

Here is a table schema:

CREATE TABLE `tag` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(45) NOT NULL COMMENT 'The tag or keyword, always lowercase would be better',
  `community_id` int(10) unsigned NOT NULL COMMENT 'The community this tag belongs to',
  `parent_id` int(10) unsigned DEFAULT NULL COMMENT 'If the tag is an alias to another tag',
  `full_description` text COMMENT 'Content describing tag',
  `short_description` tinytext COMMENT 'Short description of the tag',
  PRIMARY KEY (`id`),
  UNIQUE KEY `name_2` (`name`,`community_id`),
  KEY `FK_tag_community_idx` (`community_id`),
  KEY `FK_tag_tag_idx` (`parent_id`),
  KEY `name` (`name`),
  CONSTRAINT `FK_tag_community` FOREIGN KEY (`community_id`) REFERENCES `community` (`id`) ON DELETE CASCADE ON UPDATE NO ACTION,
  CONSTRAINT `FK_tag_tag` FOREIGN KEY (`parent_id`) REFERENCES `tag` (`id`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 COMMENT='Valid tags for a question';

I want to select Tags but if parent_id is NOT NULL, I want to return the parent instead of the child. Ideally, this query would be free of duplicates too…

Is it possible within a single query?

Server-side I use PHP Yii, if it matters…

Answer 1

use COALESE

example, (simplified query)

SELECT  COALESCE(parent_ID, iD)
FROM    tag