sql - Database normalization - who's right?

Question

My professor(who claimed to have a firm understanding about systems development for many years) and I are arguing about the design of our database.

As an example: My professor insists this design is right: (list of columns)

Subject_ID
Description
Units_Lec
Units_Lab
Total_Units

etc...

Notice the total units column. He said that this column must be included. I tried to explain that it is unnecessary, because if you want it, then just make a query by simply adding the two.

I showed him an example I found in a book, but he insists that I dont have to rely on books too much in making our system. The same thing applies to similar cases as in this one:

student_ID
prelim_grade
midterm_grade
prefinal_grade
average

ect...

He wanted me to include the average! Anywhere I go, I can find myself reading articles that convince me that this is a violation of normalization. If I needed the average, I can easily compute the three grades. He enumerated some scenarios including ('Hey! What if the query has been accidentally deleted? What will you do? That is why you need to include it in your table!')

Do I need to reconstruct my database(which consists of about more than 40 tables) to comply with what he want? Am I wrong and just have overlooked these things?

EDIT:

Another thing is that he wanted to include the total amount in the payments table, which I believe is unnecessary(Just compute the unit price of the product and the quantity.). He pointed out that we need that column for computing debits and/or credits that are critical for the overall system management, that it is needed for balancing transaction. Please tell me what you think.

Answer 1

You are right when you say your solution is more normalized.

However, there is a thing called denormalization (google for it) which is about deliberately violating normalization rules to increase queries performance.

For instance you want to retrieve first five subjects (whatever the thing would be) ordered by decreasing number or total units.

You solution would require a full scan on two tables (subject and unit), joining the resultsets and sorting the output.

Your professor's solution would require just taking first five records from an index on total_units.

This of course comes at the price of increased maintenance cost (both in terms of computational resources and development).

I can't tell you who is "right" here: we know nothing about the project itself, data volumes, queries to be made etc. This is a decision which needs to be made for every project (and for some projects it may be a core decision).

The thing is that the professor does have a rationale for this requirement which may or may not be just.

Why he hasn't explained everything above to you himself, is another question.