NULL value in multi-column primary key

Question

I\'ve got a table with several columns making up the primary key. The nature of the data stored allows some of these fields to have NULL values. I have designed

Answer 1

you can use unique keys, please take a look to this link, they work with null values

http://www.xaprb.com/blog/2009/09/12/the-difference-between-a-unique-index-and-primary-key-in-mysql/