MySQL在工作日+时间戳小时加入表，每个表计数

Question

我有几个结构完全相同的表。它们包含来自年度活动入口处填写的表格的数据。这些表中的一个字段是 DATETIME，其中包含条目的日期和时间。

出于统计目的，我试图比较每年每小时的条目数量。当然，我可以为每个表运行单独的查询并在 PHP 中将它们放在一起。但我认为在一个查询中也应该可以。但是，我不知道如何构建正确的查询。

这是我目前得到的：

SELECT
    WEEKDAY(P3.AddedOn) Day,
    HOUR(P3.AddedOn) Hour,
    COUNT(P3.AddedOn) Entries2013,
    COUNT(P2.AddedOn) Entries2012
FROM Event2013 P3
LEFT JOIN Event2012 P2
    ON WEEKDAY(P3.AddedOn) = WEEKDAY(P2.AddedOn)
        AND HOUR(P3.AddedOn) = HOUR(P2.AddedOn)
GROUP BY WEEKDAY(P3.AddedOn), WEEKDAY(P2.AddedOn), HOUR(P3.AddedOn), HOUR(P2.AddedOn)

但是这个查询会产生一些奇怪的结果，其中的数字太大并且 Entries2012 和 Entries2013 列中的数字相同。它添加了一些数据，但我无法确定到底是哪个。

我做错了什么？

感谢您的帮助！

--

用子查询解决：

SELECT
    P2.Day,
    P2.Hour,
    P2.Entries Entries2012,
    P3.Entries Entries2013
FROM
    (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2012
        GROUP BY Day, Hour
    ) P2
LEFT JOIN
    (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2013
        GROUP BY Day, Hour
    ) P3
        ON P2.Day = P3.Day
            AND P2.Hour = P3.Hour

--

现在还有某种 MySQL FULL OUTER JOIN：

SELECT
    Day,
    Hour,
    Entries2012,
    Entries2013
FROM (
    SELECT
        (CASE P2.Day
            WHEN 4 THEN "Fr"
            WHEN 5 THEN "Sat"
            WHEN 6 THEN "Sun"
            ELSE P2.Day
            END) AS Day,
        P2.Hour,
        COALESCE(P2.Entries, 0) Entries2012,
        COALESCE(P3.Entries, 0) Entries2013
    FROM (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2012
        GROUP BY Day, Hour) P2
    LEFT JOIN (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2013
        GROUP BY Day, Hour) P3
    ON P2.Day = P3.Day
        AND P2.Hour = P3.Hour
    UNION SELECT
        (CASE P3.Day
            WHEN 4 THEN "Fr"
            WHEN 5 THEN "Sat"
            WHEN 6 THEN "Sun"
            ELSE P3.Day
            END) AS Day,
        P3.Hour,
        COALESCE(P2.Entries, 0) Entries2012,
        COALESCE(P3.Entries, 0) Entries2013
    FROM (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2012
        GROUP BY Day, Hour) P2
    RIGHT JOIN (
        SELECT
            WEEKDAY(AddedOn) Day,
            HOUR(AddedOn) Hour,
            COUNT(1) AS Entries
        FROM Event2013
        GROUP BY Day, Hour) P3
    ON P2.Day = P3.Day
        AND P2.Hour = P3.Hour
    WHERE P2.Hour IS NULL) AS tmp
ORDER BY Day, Hour

Answer 1

LEFT JOIN 表示您打算从 P3 中获取所有条目，并期望其中一些在 P2 中不匹配。如果 P3 是您的基表（并且您在另一个表中查找可能的匹配项），您只需按 P3 列分组。

SELECT P3.Day Day, P3.Hour Hour, Entries2013, Entries2012 
FROM (
SELECT   
    WEEKDAY(AddedOn) Day,
    HOUR(AddedOn) Hour,
    COUNT(AddedOn) Entries2013
    GROUP BY Day, Hour
FROM Event2013) P3
LEFT JOIN (
SELECT 
    WEEKDAY(AddedOn) Day,
    HOUR(AddedOn) Hour,
    COUNT(AddedOn) Entries2012
    GROUP BY Day, Hour
FROM Event2012) P2
ON P3.DAY = P2.DAY
AND P3.Hour = P2.Hour
GROUP BY P3.Day, P3.Hour;

此外，使用 WEEKDAY 意味着您将给定年份的所有星期一、星期二等相加，然后从中获取每小时汇总。如果您想获取每日摘要（其中一天是一年中的哪一天），您应该使用 DAYOFYEAR i/o WEEKDAY。

Answer 2

查看UNION语句：http://dev.mysql.com/doc/refman/5.0/en/union.html

这结合了来自两个表的查询。