Question

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答案将我引向了CTE，但据我所知，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));

Answer 1

如何使用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) );？

{{1}}

Answer 2

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

甚至更简单，如果没有自连接，也可以检查是否可行：

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

重用相关子查询的结果

2 个答案: