Unique constraint that allows empty values in MySQL

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.

Solution 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.

Solution 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.

Solution 3:

MySQL still allows for multiple rows to have a value of NULL in a unique column.