重用相关子查询的结果

Question

我有一个表media_tags其中包含将媒体链接到用户定义的标签的两列。 我想进行一些查询，以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答案已将我引向CTE，但据我所知，它们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));

Answer 1

使用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)
                 );

Answer 2

您可以使用GROUP BY media_id进行自我连接，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

如果没有自连接，甚至更简单，请检查是否也可以：

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