@Gary just because the Force.com platform is built ON TOP of a relational database doesn't mean that it technically is one. It may have some similar constructs to a relational database, but it has some unique characteristics that make it markedly different, and also lacks some distinctive characterstics of a relational database.
If I was interviewing you and you told me that the Force.com platform is a relational database because it was built on an Oracle DB I would give you +.01 points for knowing some relatively useless trivia about the underlying architecture and -10 points for not understanding the unique characteristics and limitations of the platform.