MySQL: Unique Index Created But Cardanilty Is None Solution

So I’ve created a unique index in a table containing rows of data based on 2 columns (SKU,UPC) but the cardinality of the unique index was showing NONE after it was created. In order to solve that, here’s what I did.

It appears that you can’t create unique index based on columns which can have NULL values.

In order for unique index to work, I had to change the column properties to be a “not null” column with default value = ” (blank)

I would really love to read some comments about this, as I’m sure there must be a better explanation of why this is happening.