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You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value.
Unique fields in SQL Server are created using unique constraints or unique indexes, furthermore, each unique constraint uses a unique index. Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a single row with null value.
PRIMARY KEY constraint differs from the UNIQUE constraint in that; you can create multiple UNIQUE constraints in a table, with the ability to define only one SQL PRIMARY KEY per each table. Another difference is that the UNIQUE constraint allows for one NULL value, but the PRIMARY KEY does not allow NULL values.
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.
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