在 MySQL 中允许空值的唯一约束

我有一个存储产品代码的字段.代码是唯一的，但有些产品根本没有代码.我无法发明代码，因为那些是提供者代码.

这种约束在 MySQL 中可行吗?

我是存储过程和触发器的菜鸟，所以如果解决方案涉及其中之一，请耐心等待.

更新:该列不是 Null.这就是我无法做到这一点的原因.

解决方案

是的，您可以这样做.请参阅MySQL 参考(5.5 版).><块引用>

UNIQUE 索引创建了一个约束，使得索引中的所有值都必须是不同的.如果您尝试添加具有与现有行匹配的键值的新行，则会发生错误.对于所有引擎，UNIQUE 索引允许包含 NULL 的列有多个 NULL 值.

I have a field that stores product codes. The codes are unique, but some products simply doesn't have a code. I can't invent codes because those are providers codes.

Is this kind of constraint possible in MySQL?

I'm a noob with stored procedures and triggers, so if the solution involves one of these, please be patient.

Update: The column is NOT Null. That's why I was unable to do this.

解决方案

Yes, you can do this. See the MySQL reference (version 5.5).

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.