Question

这是一个表架构：

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';

我想选择Tags但是如果parent_id是NOT NULL，我想返回父代而不是子代。理想情况下，此查询也没有重复...

是否可以在单个查询中使用？

服务器端我使用PHP Yii，如果重要的话......

Answer 1

尝试将您的查询分为两部分：

SELECT *
FROM tag
WHERE parent_id IS NULL AND
      ...

UNION

SELECT *
FROM tag
WHERE parent_id IS NOT NULL AND
      id IN (SELECT parent_id
             FROM tag
             WHERE parent_id IS NOT NULL AND
             ...)

而不是...应该是您需要的其他过滤选项。

Answer 2

使用COALESE

示例，（简化查询）

SELECT  COALESCE(parent_ID, iD)
FROM    tag

如果不为null，MySQL选择parent而不是child

2 个答案: