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.
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.
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.
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