In a lot of web applications, like a storefront, the user's name, email, address and phone are typically persisted. A firs draft of an accounts relation would therefore have (these are attribute not SQL types)

{userid, email, name, address, phone}

{userid} and {email} are obvisous candidate keys, and address seems functionally dependent on phone, so {phone} -> {address}. I'm assuming we persist just one phone number.

So to get accounts into BCNF (and 5NF), I would decompose it into 'accounts' and 'address_info', where accounts is {userid, email, name, phone} and address_info is {phone, address}.

But is address really functionally dependenct on phone number in the 'real world' of both cell phones and landlines? A storefront application would probably have a billing address and multiple shipping address. But I'm ignoring that for now.

A storefront isn't a phone company. It doesn't care if a phone is shared by users living at the same address. It doesn't care, if the phone is shared, whether it is a shared cell (however unlikely that might be) or a shared landline. The application simply wants to avoid an update anomaly when a member's address changes.

So I'm sort of confused as to whether the FD {phone} -> {address} is really worth worrying about. It seems like it is.

thanks,
kurt

Emphatically no, there is no real-world functional dependancy of phone -> address. Mobile phones break that paradigm, and mobile professionals break it spectacularly.

I have several dozen phone numbers, most of which are virtual (no physical location at all). I have four mailing addresses in the United States, and several more abroad. I use many of those phone numbers with multiple physical addresses, and I use many of them with a single address.

I might be a rarity today, but I'd be willing to bet heavily that the trend will move more and more toward virtualization.

-PatP

PatP, thanks for the prompt reply. I appreciate it!

I guess, when thinking of functional dependencies, it really depends (excuse the pun) on who the application is for. If your Verizon, you would need to know where a landline is located. But if you Amazon you probably don't need to worry the relationship between phone number and address.