Question

我正在尝试将EXISTS语句放在EXISTS语句中，我的版本无效。有可能吗？

SELECT SUM(nbview) AS num_rows 
FROM `Release_link` 
WHERE EXISTS (SELECT id 
                 FROM Release 
                 WHERE Release.id = Release_link.release_id 
                   AND EXISTS (SELECT id 
                                 FROM Article 
                                WHERE Article.id = Release.article_id 
                                  AND Article.type = 'lol'));

感谢Jorge Campo的解决方案;）forg0t引用一些东西;）

Answer 1

问题是您在第一个Release_link上引用select并且未在第一个子选择中引用它。你可以引用两者，也可以不引用。

SELECT SUM(nbview) AS num_rows 
  FROM `Release_link` 
 WHERE EXISTS (SELECT id 
                 FROM Release 
                 WHERE Release.id = `Release_link`.release_id 
                   AND EXISTS (SELECT id 
                                 FROM Article 
                                WHERE Article.id = Release.article_id 
                                  AND Article.type = 'lol'));

或者

SELECT SUM(nbview) AS num_rows 
  FROM Release_link 
 WHERE EXISTS (SELECT id 
                 FROM Release 
                 WHERE Release.id = Release_link.release_id 
                   AND EXISTS (SELECT id 
                                 FROM Article 
                                WHERE Article.id = Release.article_id 
                                  AND Article.type = 'lol'));

对于性能问题，在这种特定情况下，更喜欢使用INNER JOIN操作。 Mysql引擎可以更好地处理索引（假设这些ID字段上有索引。）

SELECT SUM(Release_link.nbview) AS num_rows 
  FROM Release_link 
       INNER JOIN Release
          ON (Release.id = Release_link.release_id)
       INNER JOIN Article
          ON (Article.id = Release.article_id)
 WHERE Article.type = 'lol';

Mysql，嵌套EXISTS语句的正确方法是什么

1 个答案: