Question

在where条件中使用Count（）运算符时，我得到了Error Code 1111. Invalid use of group function。我认为问题在于不允许在WHERE子句中使用count运算符，但是我不确定如何使行的计数成为可用状态。

查询的目的是选择主题已观看多部电影的字段。诀窍是subject_id是带有movie_id的复合键。我在过滤主题仅看过一部电影的行时遇到问题。

查询：

SELECT 
    measures.Subject_id, 
    measures.Movie_id, 
    measures.Median_heart_rate AS Variation 
FROM measures 
WHERE COUNT(measures.Subject_id) > 1;

结果：

Subject_id     Movie_id     Variation
001            1            45
001            35           15
002            42           2        
003            1            4
003            2            5

预期结果：

Subject_id     Movie_id     Variation
001            1            45
001            35           15   
003            1            4
003            2            5

Answer 1

COUNT应该在SELECT子句中，而不在WHERE子句中。
要过滤不重复的subject_id行，可以使用带有COUNT的内部查询：

SELECT `subject_id`, 
       `movie_id`, 
       `median_heart_rate` AS Variation 
FROM   `measures` m1
WHERE  (SELECT Count(1) 
        FROM   `measures` m2
        WHERE  m2.`subject_id` = m1.`subject_id`) > 1;

Answer 2

在MySQL 8.0中，您可以使用窗口函数COUNT(...) OVER(...)来检查每个用户看过多少部电影。然后，外部查询仅根据此值过滤出记录：

SELECT *
FROM (
    SELECT 
        Subject_id, 
        Movie_id, 
        Variation,
        COUNT(*) OVER (PARTITION BY Subject_id) cnt
    FROM measures
) x
WHERE cnt > 1

Answer 3

解决方案最终如下：

SELECT measures.Subject_id, measures.Movie_id, measures.Median_heart_rate AS Variation 
FROM measures 
WHERE measures.Subject_id IN (SELECT  DISTINCT Subject_id FROM measures GROUP BY Subject_id HAVING COUNT(Subject_id) > 1);

Answer 4

我只会使用exists：

SELECT m.Subject_id, m.Movie_id, 
       m.Median_heart_rate AS Variation 
FROM measures m
WHERE EXISTS (SELECT 1
              FROM measures m2
              WHERE m2.Subject_id = m.Subject_id AND
                    m2.Movie_id <> m.Movie_id
             );

如果您的主键确实是measures(Subject_id, Movie_id)，那么它的性能也应该很好。

在WHERE子句中使用一个Count（）运算符时出现GROUP BY错误

4 个答案: