While I'd agree most people throw around the term "Excel Database" too much, and it usually makes me cringe, I'm not sure I'd agree that all data has to be on one sheet for it to be a database. Relational Database concepts say that data should be normalized into different tables so as to minimize storing the same data in multiple places. I would think each Excel worksheet could be thought of as a "table" and the data could be joined together using indexes on a "main" sheet.
Anyway, use the worksheet change event and test to see which cell has been changed like this:
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