David,

Date gives his thoughts on the use of the NULL value in logical expressions, but that doesn;t change the fact that VFP or any other product has NULL support. You need to understand NULL as it is implemented by the product you are using.

In theory, according to Codd, NULL represents the value of "I don't know", meaning that the value is not known. Whne a value is not known then any calculations or expressions using that item also have an unknown value. NULL is as simple as that and no more. The effects of NULL can be more complex though, since if the NULL value is allowed then all code that refers to the itme must account for the possibility of the NULL value.

I don't have any specific reading to recommend on this subject other than my bible "The Handbook of Relational Database Design" by Flemming and von Halle pub Addison-Wesley.