How to use foreign keys with PHP

Question

So I understand how to create foreign keys and I know what is the purpose of the FK. But I have a problem in understanding How to use them. I asked a question regarding Fore

Answer 1

Just looking at the normalization/de-normalization point:

Normalization is a process of trying to remove redundancy in your database - in your example the name field in user_purchase is redundant - I can find out the name of the item by looking it up in the items table using i_id.

So if we were to look at normalizing user_purchase we'd probably remove the name field and use a JOIN to retrieve that when we needed it. This would, of course, also mean we don't need the second FOREIGN KEY reference to items.

De-normalization is basically going the opposite way - adding redundancy - usually done for performance reasons.

However, in your example you might also consider de-normalization for business reasons too. For example you might decide it is important to store the product name as it was when the user actually purchased it (rather than what it's called now) - just in case you need to be able to re-print an invoice for example. However even in this case you wouldn't want the FOREIGN KEY back to items (as it would "break" if the product was re-named).