重用相关子查询的结果

Posted 2023-04-14

【中文标题】重用相关子查询的结果

【英文标题】：Reuse result of a correlated subquery

【发布时间】：2019-07-22 21:30:13

【问题描述】：

我有一个表media_tags，其中包含两列将媒体链接到用户定义的标签。我想做一些查询，选择标签上条件复杂的媒体。我当前的解决方案是多次使用相同的相关子查询。

多个 SO 答案已将我指向 CTE，但据我了解，这些只能用于替换不相关的子查询。

SELECT outer.media_id
FROM media_tags AS outer
WHERE 1 IN (SELECT tag_id FROM media_tags AS inner WHERE outer.media_id = inner.media_id)
AND 2 NOT IN (SELECT tag_id FROM media_tags AS inner WHERE outer.media_id = inner.media_id)
AND (3 IN (SELECT tag_id FROM media_tags AS inner WHERE outer.media_id = inner.media_id)
OR 4 IN (SELECT tag_id FROM media_tags AS inner WHERE outer.media_id = inner.media_id));

【参考方案1】：

您可以进行自联接，GROUP BY media_id 并在HAVING 子句中设置条件：

SELECT outer.media_id
FROM media_tags AS outer INNER JOIN media_tags AS inner
ON outer.media_id = inner.media_id
GROUP BY outer.media_id
HAVING 
  SUM(inner.tag_id = 1) > 0
  AND
  SUM(inner.tag_id = 2) = 0
  AND
  SUM(inner.tag_id IN (3, 4)) > 0

或者更简单，如果没有自连接检查这是否也有效：

SELECT media_id
FROM media_tags
GROUP BY media_id
HAVING 
  SUM(tag_id = 1) > 0
  AND
  SUM(tag_id = 2) = 0
  AND
  SUM(tag_id IN (3, 4)) > 0

【参考方案2】：

用not exists怎么样？

SELECT m2.media_id
FROM media_tags mt
WHERE NOT EXISTS (SELECT tag_id
                  FROM media_tags mt2
                  WHERE mt2.media_id = mt.media_id AND
                        mt2.tag_id IN (1, 2, 3, 4)
                 );

【讨论】：

我没有看到将它用于复杂查询的方法，例如 tag_1 AND NOT tag_2 AND (tag_3 OR tag_4)