php - MySQL Join 返回超过预期

Question

这个问题让我睡了 2 天。

我有两张 table

views 

id | postid |   date     | count
=================================  
13 |   8    | 2016-07-16 |  38
16 |   8    | 2016-07-17 |  35
15 |   9    | 2016-07-16 |  7
17 |   9    | 2016-07-17 |  32
14 |   12   | 2016-07-16 |  17
18 |   12   | 2016-07-17 |  13


visitors 

id | postid |   date     | ip
=================================  
13 |   8    | 2016-07-16 |  127.0.0.1
17 |   8    | 2016-07-17 |  127.0.0.1
18 |   8    | 2016-07-17 |  127.0.0.1
16 |   9    | 2016-07-16 |  127.0.0.1
19 |   9    | 2016-07-17 |  127.0.0.1
14 |   12   | 2016-07-16 |  127.0.0.1
15 |   12   | 2016-07-16 |  127.0.0.1
20 |   12   | 2016-07-17 |  127.0.0.1
21 |   12   | 2016-07-17 |  127.0.0.1

和下面的查询

$query = $wpdb->get_results("
    SELECT
        SUM(a.count) AS countviews, 
        COUNT(b.ip) AS countvisitors,
        a.postid
    FROM views a
    RIGHT JOIN visitors b
        ON a.postid=b.postid
        AND a.date=b.date
    WHERE 
        a.date
    BETWEEN 
        DATE_SUB('2016-07-17', INTERVAL 3 DAY) 
    AND 
        '2016-07-17' 
    GROUP BY 
        a.postid
    ORDER BY 
        countviews DESC
");

当我 print_r 输出时，我会看到以下结果

Array
(
    [0] => stdClass Object
        (
            [countviews] => 108
            [countvisitors] => 3
            [postid] => 8
        )

    [1] => stdClass Object
        (
            [countviews] => 60
            [countvisitors] => 4
            [postid] => 12
        )

    [2] => stdClass Object
        (
            [countviews] => 39
            [countvisitors] => 2
            [postid] => 9
        )

 )

只有 [countviews] 结果高于预期。我要数一下，看看来自 postid 8 的 countviews 一定不是“108”，而是“73”。奇怪的是 postid 8 的最后计数是“35”。 “108”减去“35”=“73”。所以 View 表是双倍的？

RIGHT JOIN、LEFT JOIN 和 INNER JOIN 给出的结果都是一样的。

Answer 1

如果要计数，则不能在此处进行连接。您建立的关系是创建 View 表的倍数，以防同一 postid 在您的搜索参数中有多个日期。

您可以通过使用子查询来避免这种情况:

SELECT
    SUM(a.count) AS countviews,
    (SELECT COUNT(b.ip) FROM visitors i WHERE b.date BETWEEN DATE_SUB("2016-07-17", INTERVAL 3 DAY) AND "2016-07-17" AND i.postid = a.postid) AS countvisitors,
    a.postid
FROM views a
WHERE 
    a.date
BETWEEN 
    DATE_SUB('2016-07-17', INTERVAL 3 DAY) 
AND 
    '2016-07-17' 
GROUP BY 
    a.postid
ORDER BY 
    countviews DESC

希望我做对了。让我知道这是否有帮助:)