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 |
+-------------+------------+----------+----------+
这样我的回报就像这样
+-------------+------------+----------+----------+
| 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 |
+-------------+------------+----------+----------+
这里的全部想法是能够将所有尚未notes应用于groupdate匹配history table's {{ 1}}。只有在发布便笺时，attend_date才会设置为groupdate。然而，正如我所知，查询为所有四个attend_date而不是仅一个groupdate返回2016-01-26 clients。在此先感谢您的帮助。
答案 0 :(得分：1)
查看您的表格结构，我看到您尝试加入的attend_date和groupdate之间的关系，但我也看到这些表格有clientid和{{1共同的。
由于您只加入了该日期，因此您将获得所有客户的笛卡尔产品以及该条件的所有日期。您可以通过添加逻辑memberid的附加条件来使联接的ON子句更具体。
AND如果您想要的结果集还要求条件在这些表之间匹配LEFT JOIN participation AS p
ON groupdate = attend_date
AND h.clientid = p.clientid
，那么对我来说并不是完全明显的，但如果确实如此，那么解决方案就像另一个memberid一样简单
AND有助于不将联接的LEFT JOIN participation AS p
ON groupdate = attend_date
AND h.memberid = p.memberid
子句视为常见列之间的匹配，尽管这是最常用的方式。相反，ON子句只需要提供一些条件，即当返回连接行时返回true（就像你习惯使用ON子句一样）。这意味着你可以放置任何可以被评估为真或假的东西，允许复杂的连接逻辑。 WHERE和ORDER BY也可以这样说。