我正在寻找如何实现对NULL值进行唯一性约束的方法。
MySQL不应允许多个NULL值。
员工:
MySQL不应允许多个NULL值。
员工:
id | name
---|-----
1 | null
2 | null -> should give error during inserting 2nd row.
id | name
---|-----
1 | null
2 | null -> should give error during inserting 2nd row.
MySQL 5.7提供了一种解决方法:
mysql> CREATE TABLE `null_test` (
-> `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
-> `const` varchar(255) NOT NULL DEFAULT '',
-> `deleted_at` datetime DEFAULT NULL,
-> PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.03 sec)
使用软删除时,最好每个约束条件只有一行数据,deleted_at=NULL
。
mysql> ALTER TABLE `null_test` ADD `vconst` int(1) GENERATED ALWAYS AS (((NULL = `deleted_at`) or (NULL <=> `deleted_at`))) VIRTUAL;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
所以我创建了一个虚拟列,当deleted_at
被设置时,它会从1
翻转到null
。
mysql> ALTER TABLE `null_test` ADD UNIQUE KEY `nullable_index` (`const`,`vconst`);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
不要将deleted_at
包含在唯一约束中,而是添加虚拟列vconst
。
mysql> INSERT INTO `null_test` SET `const` = 'Ghost';
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM `null_test` WHERE `const` = 'Ghost';
+--------+-------+------------+--------+
| id | const | deleted_at | vconst |
+--------+-------+------------+--------+
| 999901 | Ghost | NULL | 1 |
+--------+-------+------------+--------+
1 row in set (0.01 sec)
vconst
(但无论如何都不能插入)。mysql> INSERT INTO `null_test` SET `const` = 'Ghost';
ERROR 1062 (23000): Duplicate entry 'Ghost-1' for key 'nullable_index'
再次插入会引发重复条目错误。
mysql> UPDATE `null_test` SET `deleted_at` = NOW() WHERE `const` = 'Ghost';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
设置delete_at
与之相同,无需触碰vconst
,它将自动翻转。
mysql> SELECT * FROM `null_test` WHERE `const` = 'Ghost';
+--------+-------+---------------------+--------+
| id | const | deleted_at | vconst |
+--------+-------+---------------------+--------+
| 999901 | Ghost | 2017-02-16 22:07:45 | NULL |
+--------+-------+---------------------+--------+
1 row in set (0.00 sec)
mysql> INSERT INTO `null_test` SET `const` = 'Ghost';
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM `null_test` WHERE `const` = 'Ghost';
+--------+-------+---------------------+--------+
| id | const | deleted_at | vconst |
+--------+-------+---------------------+--------+
| 999901 | Ghost | 2017-02-16 22:07:45 | NULL |
| 999903 | Ghost | NULL | 1 |
+--------+-------+---------------------+--------+
2 rows in set (0.01 sec)
deleted_at
,您可能希望将 deleted_at
包含在索引中,或者使用新索引,但我会让我的负载测试来决定。https://mariadb.com/kb/en/sql-99/constraint_type-unique-constraint/
一个唯一约束条件使得不可能提交任何操作,该操作会导致唯一键包含任何非空重复值。(多个空值是允许的,因为空值从未等于任何东西,甚至是另一个空值。)NOT NULL
并禁止任何行具有NULL。alter table yourtable add column `virtual_null` varchar(20) GENERATED ALWAYS AS (if(isnull(`your_nullable_column`),'null',`your_nullable_column`))) VIRTUAL;
alter table yourtable add constraint unique(virtual_null);
做到这一点,你会很开心的,在幕后,MySQL的null是一个哈希值。因为这样就不可能比较两个null值...
抱歉我的英语不好,祝你好运