mysql 需要从查询中提取中值

Question

我有以下查询，我需要从中提取 total_views 的中值。

    SELECT 
    @rownum:=@rownum + 1 AS row_num, total_views, projectId
FROM
    (SELECT 
        a.creation,
            a.projectId,
            devices,
            browserIds,
            devices + browserIds AS total_views
    FROM
        ((SELECT 
        projectId, creation
    FROM
        event
    WHERE
        kind = 'project_creation'
            AND creation > '2017-04-28') a
    INNER JOIN ((SELECT 
        COUNT(DISTINCT deviceId) AS devices, projectId, creation
    FROM
        event
    WHERE
        kind = 'open' AND component = 'mobile'
    GROUP BY projectId) b
    JOIN (SELECT 
        COUNT(DISTINCT browserId) AS browserIds, projectId, creation
    FROM
        event
    WHERE
        kind = 'open' AND component = 'web'
    GROUP BY projectId) c ON b.projectId = c.projectId) ON a.projectId = b.projectId
        OR a.projectId = c.projectId)
    ORDER BY total_views ASC) d,
    (SELECT @rownum:=0) e
;

这是结果的一部分：

1   1   151
2   1   256
3   1   301
4   2   404
5   2   305
6   3   895
7   4   654
8   4   369
9   9   874
10  10  123

我需要扩展查询以提取 total_views 的中值。有什么想法吗？

Answer 1

找到解决方案，需要使用@rownum变量的值而不是使用字段row_num的值来确定中间值的位置。然后我计算结果集中间的 total_views 的平均值。（如果结果的行数为偶数，则取两个中间值的平均值。如果结果集的行数为奇数，则取中间值的平均值，这与中间值相同）。因此使用条件：

WHERE row_num in (CEIL(@rownum/2), FLOOR(@rownum/2))

完整查询：

SELECT avg(total_views) from 
(SELECT 
    @rownum:=@rownum + 1 AS row_num, total_views, projectId
FROM
    (SELECT 
        a.creation,
            a.projectId,
            devices,
            browserIds,
            devices + browserIds AS total_views
    FROM
        ((SELECT 
        projectId, creation
    FROM
        event
    WHERE
        kind = 'project_creation'
            AND creation > '2017-04-28') a
    INNER JOIN ((SELECT 
        COUNT(DISTINCT deviceId) AS devices, projectId, creation
    FROM
        event
    WHERE
        kind = 'open' AND component = 'mobile'
    GROUP BY projectId) b
    JOIN (SELECT 
        COUNT(DISTINCT browserId) AS browserIds, projectId, creation
    FROM
        event
    WHERE
        kind = 'open' AND component = 'web'
    GROUP BY projectId) c ON b.projectId = c.projectId) ON a.projectId = b.projectId
        OR a.projectId = c.projectId)
    ORDER BY total_views ASC) d,
    (SELECT @rownum:=0) e) f WHERE row_num in (CEIL(@rownum/2), FLOOR(@rownum/2))
;

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