COUNT個最高的MySQL GROUP BY字段（基於JOIN）

Question

SELECT COUNT(dev_link.id) c, pattern.direction, pattern.section, (
    SELECT to_stop AS `stop`
    FROM dev_link d
    WHERE d.section = section
    ORDER BY d.sequence DESC
    LIMIT 1
) code
FROM pattern
INNER JOIN dev_link ON dev_link.section = pattern.section
WHERE pattern.service = "YSEO252"
GROUP BY pattern.code
ORDER BY c DESC

上面是一個查詢及其結果。 當前，這選擇了太多數據，我需要將其縮小到以下范圍：

第一行的方向為outbound ，另一行的方向為inbound 。 如果僅僅是我可以輕松地按方向GROUP BY 。 但是，我需要選擇具有最高c值的inbound和outbound行。 因此，將根據第一行和第三行上方的結果進行選擇。

如何更改查詢以使其返回這些行？

Answer 1

我將使用變量來模擬row_number（）

SELECT * FROM (
    SELECT * , 
        @rn := if(@prevDirection = direction, @rn + 1, 1) as rn,
        @prevDirection := direction,
    FROM (
        SELECT COUNT(dev_link.id) c, pattern.direction, pattern.section, (
            SELECT to_stop AS `stop`
            FROM dev_link d
            WHERE d.section = section
            ORDER BY d.sequence DESC
            LIMIT 1
        ) code
        FROM pattern
        INNER JOIN dev_link ON dev_link.section = pattern.section
        WHERE pattern.service = "YSEO252"
        GROUP BY pattern.code
    ) t1 ORDER BY direction, c DESC
) t1 WHERE rn = 1

Answer 2

( SELECT ... WHERE ... AND pattern.direction='inbound' ... LIMIT 1)
UNION ALL
( SELECT ... WHERE ... AND pattern.direction='outbound' ... LIMIT 1 );

否則，每個SELECT都將與原始SELECT相似。

Answer 3

您只需要在group by子句中另外使用MAX函數，例如：

SELECT MAX(c)
FROM mytable
GROUP BY direction

Answer 4

您必須使用having子句。

SELECT COUNT(dev_link.id) c, pattern.direction, pattern.section, (
    SELECT to_stop AS `stop`
    FROM dev_link d
    WHERE d.section = section
    ORDER BY d.sequence DESC
    LIMIT 1
) code
FROM pattern
INNER JOIN dev_link ON dev_link.section = pattern.section
WHERE pattern.service = "YSEO252"
GROUP BY pattern.code
HAVING COUNT(dev_link.id) = MAX(COUNT(dev_link.id))
ORDER BY c DESC