重用相關子查詢的結果

Question

我有一個表media_tags其中包含將媒體鏈接到用戶定義的標簽的兩列。 我想進行一些查詢，以選擇標簽上條件復雜的媒體。 我當前的解決方案是多次使用相同的相關子查詢。

多個SO答案已將我引向CTE，但據我所知，它們只能用於替換不相關的子查詢。

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怎么樣？

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進行自我連接，並在HAVING子句中設置條件：

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

如果沒有自連接，甚至更簡單，請檢查是否也可以：

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