Entity relation (normal forms)

Question

Let's assume we have table USERS like:

..ID..username

1. user1
2. user2
3. user3

Users can have Bills ( user -> have_many -> bill relation).

Table BILLS like:

..ID..user_id

1. 1
2. 2
3. 2

Also we have Products so every Product can be associated to ONLY ONE bill ( product -> has_one -> bill relation).

Table PRODUCTS like:

..ID..bill_id

1. 2
2. 3
3. 1

So, as you can see, our user can have lot of products (through bills).

My question:

Would it be correct DUE TO Database normalization to add second foreign key to PRODUCTS table named user_id to quickly select all user's Products from PRODUCTS table, or it's not correct and I should use JOIN statement to select all User's Products?

PS Sorry for dirty tables drawing )

Answer 1

I would rather go with the normalized view (where you DO NOT have a user_id in the products table).

The only time I would ever consider this, as a last option, is if the performance REALY requires it.

Answer 2

It is usually better to normalize data so as not to duplicate information and retain data consistency.

However there are exceptions required in real life systems, often for performance reasons when dealing with huge volumes of data.