使用唯一键时重复数据

Question

对于我的表，我在 activity_id-actor_id-end_date 上定义了唯一索引；

mysql> show keys from sg_activity_property;
+----------------------+------------+-------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table                | Non_unique | Key_name    | Seq_in_index | Column_name          | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------------------+------------+-------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+
| sg_activity_property |          0 | PRIMARY     |            1 | activity_property_id | A         |         506 |     NULL | NULL   |      | BTREE      |         |
| sg_activity_property |          0 | activity_id |            1 | activity_id          | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| sg_activity_property |          0 | activity_id |            2 | actor_id             | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| sg_activity_property |          0 | activity_id |            3 | end_date             | A         |        NULL |     NULL | NULL   | YES  | BTREE      |         |
+----------------------+------------+-------------+--------------+----------------------+-----------+-------------+----------+--------+------+------------+---------+
4 rows in set (0.00 sec)

那么，这些数据怎么可能存在呢？

mysql> SELECT activity_property_id, activity_id, actor_id, start_date, end_date FROM  `sg_activity_property` WHERE  `activity_id` =250;
+----------------------+-------------+----------+---------------------+----------+
| activity_property_id | activity_id | actor_id | start_date          | end_date |
+----------------------+-------------+----------+---------------------+----------+
|                  509 |         250 |        8 | 2011-09-02 11:10:50 | NULL     |
|                  510 |         250 |        8 | 2011-09-02 11:10:50 | NULL     |
+----------------------+-------------+----------+---------------------+----------+
2 rows in set (0.00 sec)

编辑：这是SHOW CREATE TABLE sg_activity_property的输出：

mysql> SHOW CREATE TABLE sg_activity_property;
+----------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table                | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
+----------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| sg_activity_property | CREATE TABLE `sg_activity_property` (
  `activity_property_id` int(10) unsigned NOT NULL auto_increment,
  `activity_id` int(10) unsigned NOT NULL,
  `actor_id` int(10) unsigned NOT NULL,
  `importance` enum('very low','low','normal','high','very high') NOT NULL default 'normal',
  `urgency` enum('!','!!') default NULL,
  `completed` tinyint(1) NOT NULL,
  `start_date` datetime NOT NULL,
  `end_date` datetime default NULL,
  `review_frequency` int(11) NOT NULL default '1',
  `review_frequency_unit` enum('day','week','month','quarter','year') NOT NULL default 'week',
  PRIMARY KEY  (`activity_property_id`),
  UNIQUE KEY `activity_id` (`activity_id`,`actor_id`,`end_date`)
) ENGINE=MyISAM AUTO_INCREMENT=511 DEFAULT CHARSET=latin1 |
+----------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.19 sec)

Answer 1

这是预期的行为。查看MySQL文档：
http://dev.mysql.com/doc/refman/5.0 /en/create-table.html

UNIQUE 索引创建一个约束，使得索引中的所有值
必须是独特的。如果您尝试添加带有以下内容的新行，则会发生错误
与现有行匹配的键值。此限制不适用
为 NULL 值（BDB 存储引擎除外）。对于其他发动机，
UNIQUE 索引允许列有多个 NULL 值，这些值可以包含
空。

Answer 2

由于 end_date 上的 NULL，
从技术上讲，NULL <> EMPTY 或任何值只是一个缺少值的占位符，

因此，将其更改为 NOT NULL 应该修复

PS: 当您执行更改时

alter table sg_activity_property 
modify column end_date datetime not null
default '0000-00-00 00:00:00';

，此操作将失败，因为 mysql 会尝试将 NULL 转换为 <代码>0000-00-00 00:00:00,
为了解决这个问题，您可以先为其分配一些随机值，
或者只是简单地删除重复项之一

Answer 3

end_date 中有 NULL 值。

NULL 值是未定义的值，因此有两个
NULL 值并不相同。