Question

在我的MySql中，我有两个表：tblN1和tblN2。

表tblN1是一张旧表，在网站上的受限区域内存储了帐户。

表tblN2是一个新表，在网站上的受限区域内存储了帐户。

如果在tblN1上尝试此查询：

mysql> SELECT
    COUNT(*) AS TotalN1
FROM
    `tblN1`
WHERE
    UserN1 IN ('7047505')
AND dateN1 BETWEEN MAKEDATE(YEAR(CURDATE()), 1)
AND CURDATE();
+---------+
| TotalN1 |
+---------+
|       4 |
+---------+
1 row in set

此查询的返回值为4。

如果在tblN2上尝试此查询：

mysql> SELECT
    COUNT(*) as TotalN2
FROM
    `tblN2`
WHERE
    UserN2 IN ('7047505')
AND dateN2 BETWEEN MAKEDATE(YEAR(CURDATE()), 1)
AND CURDATE();
+---------+
| TotalN2 |
+---------+
|       0 |
+---------+
1 row in set

此查询的返回值为0。

我试图加入两个表来获取用户的总访问次数。

mysql> SELECT
    (
        COUNT(DISTINCT A.ID) + COUNT(DISTINCT CA.ID)
    ) AS SuperTotal,
    COUNT(DISTINCT A.ID) AS TotalN1,
    COUNT(DISTINCT CA.ID) AS TotalN2,
    UserN1 AS UserN1,
    UserN2  AS UserN2
FROM
    `tblN1` A
JOIN `tblN2` CA ON A.UserN1 = CA.UserN2
WHERE
    CA.UserN2 IN ('7047505')
AND (
    dateN1 BETWEEN MAKEDATE(YEAR(CURDATE()), 1)
    AND CURDATE()
    AND `dateN2` BETWEEN MAKEDATE(YEAR(CURDATE()), 1)
    AND CURDATE()
);
+------------+---------+---------+--------+--------+
| SuperTotal | TotalN1 | TotalN2 | UserN1 | UserN2 |
+------------+---------+---------+--------+--------+
|          0 |       0 |       0 | NULL   | NULL   |
+------------+---------+---------+--------+--------+
1 row in set

所有字段的返回值为0，为什么？

请帮帮我，谢谢你。

编辑＃1

mysql> SELECT sum(total) as Total FROM
    (SELECT COUNT(*) AS Total
    FROM `tblN1`
    WHERE UserN1 IN ('7047505')
          AND dateN1 BETWEEN MAKEDATE(YEAR(CURDATE()), 1) AND CURDATE()
    UNION ALL
    SELECT COUNT(*)
    FROM `tblN2`
    WHERE UserN2 IN ('7047505')
          AND dateN2 BETWEEN MAKEDATE(YEAR(CURDATE()), 1) AND CURDATE()
    ) AS t;


+-------+
| Total |
+-------+
| 0     |
+-------+
1 row in set

Answer 1

连接会导致返回0。从您的示例中可以清楚地看出，给定用户没有满足第二个表中where条件的任何记录。当您在第二个表上加入第一个用户ID表时，第二个表中没有返回任何结果，因此该连接将从第一个表中删除相应的记录。

更新：

不是使用join，而是在子查询中将2个查询与union结合起来，并将它们添加到外部查询中：

SELECT sum(total) as Total FROM
    (SELECT COUNT(*) AS Total
    FROM `tblN1`
    WHERE UserN1 IN ('7047505')
          AND dateN1 BETWEEN MAKEDATE(YEAR(CURDATE()), 1) AND CURDATE()
    UNION ALL
    SELECT COUNT(*)
    FROM `tblN2`
    WHERE UserN2 IN ('7047505')
          AND dateN2 BETWEEN MAKEDATE(YEAR(CURDATE()), 1) AND CURDATE()
    ) AS t

加入两个表以获取MySql中用户的总访问次数

1 个答案: