Primary record in hash table

I've got units and categories. A unit may be in multiple categories. Thus, there are three tables: Unit, Category, and Unit_Category.

One of these categories must be the primary category. How would I mark that, and constraint it properly. I was thinking to use a field in the hash table to mark the primary category. But how would I constraint it to only have one primary category per unit?

Brian Received on Mon Nov 20 2000 - 17:33:52 CST