MySQL 查询没有产生正确的结果

Question

鉴于此:

MariaDB [master]> describe history;
+--------------+------------------+------+-----+---------+----------------+
| Field        | Type             | Null | Key | Default | Extra          |
+--------------+------------------+------+-----+---------+----------------+
| historyid    | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| amount       | float            | NO   |     | NULL    |                |
| subsidy      | char(1)          | NO   |     | NULL    |                |
| last_payment | date             | NO   |     | NULL    |                |
| amount_paid  | float            | NO   |     | NULL    |                |
| balance      | float            | NO   |     | NULL    |                |
| attend       | char(1)          | NO   |     | N       |                |
| attend_date  | date             | NO   |     | NULL    |                |
| groupid      | int(11) unsigned | NO   |     | NULL    |                |
| clientid     | int(10) unsigned | NO   | MUL | NULL    |                |
| memberid     | int(10) unsigned | NO   | MUL | NULL    |                |
+--------------+------------------+------+-----+---------+----------------+


MariaDB [master]> describe participation;
+-----------+------------------+------+-----+---------+----------------+
| Field     | Type             | Null | Key | Default | Extra          |
+-----------+------------------+------+-----+---------+----------------+
| partid    | int(11)          | NO   | PRI | NULL    | auto_increment |
| notes     | varchar(255)     | NO   |     | NULL    |                |
| groupdate | date             | NO   |     | NULL    |                |
| clientid  | int(10) unsigned | NO   | MUL | NULL    |                |
| memberid  | int(10) unsigned | NO   | MUL | NULL    |                |
+-----------+------------------+------+-----+---------+----------------+

MariaDB [master]> select * from participation;
+--------+-----------+------------+----------+----------+
| partid | notes     | groupdate  | clientid | memberid |
+--------+-----------+------------+----------+----------+
|    194 | test test | 2016-01-26 |        3 |        1 |
+--------+-----------+------------+----------+----------+

如何编写以下查询

MariaDB [master]> SELECT attend_date, groupdate, h.clientid, h.memberid
-> FROM history AS h
-> LEFT JOIN participation AS p ON groupdate = attend_date
-> WHERE h.memberid = "1"
-> AND MONTH(attend_date) = "1"
-> AND YEAR(attend_date) = "2016"
-> AND attend_date <> "0000-00-00"
-> ORDER BY attend_date ASC;
+-------------+------------+----------+----------+
| attend_date | groupdate  | clientid | memberid |
+-------------+------------+----------+----------+
| 2016-01-26  | 2016-01-26 |        3 |        1 |
| 2016-01-26  | 2016-01-26 |        4 |        1 |
| 2016-01-26  | 2016-01-26 |        1 |        1 |
| 2016-01-26  | 2016-01-26 |        2 |        1 |
| 2016-01-28  | NULL       |        3 |        1 |
| 2016-01-28  | NULL       |        4 |        1 |
| 2016-01-28  | NULL       |        1 |        1 |
| 2016-01-28  | NULL       |        2 |        1 |
+-------------+------------+----------+----------+

所以我的返回是这样的

+-------------+------------+----------+----------+
| attend_date | groupdate  | clientid | memberid |
+-------------+------------+----------+----------+
| 2016-01-26  | 2016-01-26 |        3 |        1 |
| 2016-01-26  | NULL       |        4 |        1 |
| 2016-01-26  | NULL       |        1 |        1 |
| 2016-01-26  | NULL       |        2 |        1 |
| 2016-01-28  | NULL       |        3 |        1 |
| 2016-01-28  | NULL       |        4 |        1 |
| 2016-01-28  | NULL       |        1 |        1 |
| 2016-01-28  | NULL       |        2 |        1 |
+-------------+------------+----------+----------+

这里的整个想法是能够拉动 所有 尚未将 notes 应用到 groupdate 匹配 历史表的 attend_date。并且只有在发布注释时才会将 groupdate 设置为 attend_date。然而，正如我所知道的那样，查询为所有四个 clients 返回 groupdate 的 2016-01-26 而不仅仅是一个。在此先感谢您的帮助。

Answer 1

查看您的表结构，我看到您的 attend_date 和您尝试加入的 groupdate 之间的关系，但我还看到那些表有 clientid 和 memberid 相同。

由于您仅在日期加入，因此您将返回该条件下所有客户和所有日期的笛卡尔积。您可以通过使用逻辑 AND 添加附加条件来使连接的 ON 子句更加具体。

LEFT JOIN participation AS p
  ON groupdate = attend_date
  AND h.clientid = p.clientid

如果你想要的结果集也需要一个条件来匹配这些表之间的 memberid 对我来说不是很明显，但如果是这样，解决方案就像另一个 AND 一样简单。

LEFT JOIN participation AS p
  ON groupdate = attend_date
  AND h.memberid = p.memberid

不要将连接的 ON 子句仅仅看作是公共(public)列之间的匹配，尽管这是它最常见的使用方式。相反，ON 子句只需要提供一些条件，当 true 导致返回连接的行时(就像您习惯使用 WHERE 子句一样)。这意味着您可以在那里放置任何可以评估为真或假的东西，从而允许复杂的连接逻辑。 ORDER BY 和 GROUP BY 也是如此。