Hi,

first time post here...

Trying to help a friend out, but struggling to articulate why design is poor.

She has set up her database where each company has a set of tables (each set are identical but uniquely named).

I initially thought this was not normalized, but questioning that assumption now. A single group of tables is normalized...but she has lots of groups of these tables. Can't find one of the 1/2/3/4 Normal form rules that it breaks. Am I missing something?

I'm struggling to communicate WHY she should go back and "normalize" everything.

Can anyone help articulate this?

thanks!

ask her to write a query which obtains the average sales for all companies, or even just the company with the largest sales amount

A database with multiple identical tables won't necessarily violate any Normal Form. It probably will violate the Principle of Orthogonal Design however. You ought to avoid such designs because they give rise to ambiguous, potentially inconsistent data and lead to redundancy and complexity in your data manipulation code.