问题
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.
回答1:
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.
回答2:
Yes, if you make the product code column nullable (not declared with NOT NULL), the unique key will allow multiple rows with NULL product codes.
回答3:
MySQL still allows for multiple rows to have a value of NULL in a unique column.
来源：https://stackoverflow.com/questions/1346765/unique-constraint-that-allows-empty-values-in-mysql