使用唯一键时复制数据

Question

对于我的表，我已经定义了一个唯一索引activity_id - actor_id - end_date;

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，这个数据如何存在？

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：这里的SHOW CREATE TABLE sg_activity_property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

因为NULL上END_DATE的，
技术上NULL <> EMPTY或任意数值，只是一个占位符，其中值缺失

因此，将其更改为NOT NULL应修复

PS：当你在做改变

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

这将失败，因为MySQL会尝试将NULL转换为0000-00-00 00:00:00，
为了解决这个问题，你可以一些随机值首先分配给它，
或者只是简单地删除其中一个副本

Answer 2

您在end_date中有NULL值。

NULL值是未定义的值，因此两个 NULL值不相同。

Answer 3

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

唯一索引创建，使得该指数 所有的值必须是不同的约束。如果尝试使用与现有行匹配的键值 添加新行，则会发生错误。除了BDB存储引擎之外，此限制不适用于 空值。对于其他引擎， UNIQUE索引允许针对可包含空值的列的多个NULL值。