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mysql 中存在null和空时创建唯一索引的方法

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Release: 2016-06-07 16:27:56
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好多情况下数据库默认值都有null,但是经过程序处理很多时候会出现,数据库值为空而不是null的情况。此时创建唯一索引时要注意了,此时数据库会把空作为多个重复值,而创建索引失败,示例如下: 步骤1: mysql select phone ,count(1) from User group by ph

好多情况下数据库默认值都有null,但是经过程序处理很多时候会出现,数据库值为空而不是null的情况。此时创建唯一索引时要注意了,此时数据库会把空作为多个重复值,而创建索引失败,示例如下:

步骤1:

mysql> select phone ,count(1) from User group by phone;
+-----------------+----------+
| phone | count(1) |
+-----------------+----------+
| NULL | 70 |
| | 40 |
| +86-13390889711 | 1 |
| +86-13405053385 | 1 |

步骤一中发现数据库中有70条null数据,有40条为空的数据。

步骤2:

mysql> select count(1) from User where phone is null;
+----------+
| count(1) |
+----------+
| 70 |
+----------+
1 row in set (0.00 sec)

经2再次验证数据库中null和空不一样的两个值。

步骤3:

mysql> alter table User add constraint uk_phone unique(phone);
ERROR 1062 (23000): Duplicate entry '' for key 'uk_phone'
此时创建索引提示‘ '为一个重复的属性。

步骤4:将所有的空值改成null

mysql> update User set phone = NULL where phone = '';
Query OK, 40 rows affected (0.11 sec)
Rows matched: 40 Changed: 40 Warnings: 0
步骤5:再次创建唯一索引

mysql> alter table User add constraint uk_phone unique(phone);
Query OK, 0 rows affected (0.34 sec)
Records: 0 Duplicates: 0 Warnings: 0

创建成功,OK了

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