Ok now I'm gonna tell you about my "Databases 2" exam. This is gonna be long.

I'd like to know if DB designers actually have this workflow. I'm gonna "challenge" the reader, but I'm not playing smartass. The mistakes I point out here are MY mistakes.

So, in my uni there's this course, "Databases 2" ("Databases 1" is relational algebra and theoretical stuff), which consist in one exercise: design a SQL database.

We get the description of a system. Almost a two pages pdf. Of course it could be anything. Here I'm going to pretend the project is a YouTube clone (it's one of the practice exercises).

We start designing a ER diagram that describes the system. It must be fucking accurate: e.g. if we describe a "view" as a relationship between the entities User and Video, it MUST have at least another attribute, e.g. the datetime, even if the description doesn't say it. The official reason?

"The ER relationship describes a set of couples. You can not have two elements equal, thus if you don't put any attribute, it means that any user could watch a video only once. So you must put at least something else."

Do you get my point? In this phase we're not even talking about a "database", this is an analysis phase.

Then we describe the type dictionary. So far so good, we just have to specify the type of any attribute.

And now... Constraints.
Oh my god the constraints. We have to describe every fucking constraint of our system. In FIRST ORDER LOGIC. Every entity is a set, and Entity(e) means that an element e belongs to the set Entity. "A user must leave a feedback after he saw a video" becomes like

For all u,v,dv,df,f ( User(u) and Video(v) and View(u, v, dv) and feedback(u, v, f) ) ---> dv < df

provided that dv and df are the datetimes of the view and the feedback creation (it is clear in the exercise, here seems kinda cryptic)

Of course only some of the constraints are explicitly described. This one, for example, was not in the text. If you fail to mention any "hidden" constraint, you lose a lot of points. Same thing if you not describe it correctly.

Now it's time for use cases.
You start with the usual stickman diagram. So far so good.
Then you have to describe their main functions.

In first order logic. Yes.

So, if you got the point, you may think that the following is correct to get "the average amount of feedback values on a single video" (1 to 5, like the old YT).
(let's say that feedback is a relationship with attribute between User and Video

getAv(Video v): int
Let be F = { va | feedback(v, u, va) } for any User u
Let av = (sum forall f in F) / | F |
return av

But nope, there's an error here. Can you spot it (I didn't)?

F is a set. Sets do not have duplicates! So, the F set will lose some feedback values! I can not define that as a simple set!

It has to be a set of couples, like (v, u), where v is the value and u the user; this way we can have duplicate feedback values in our set.

This concludes the analysis phase. Now, the design.

Well we just refactor everything we have done until now. Is-a relations become relationships, many-to-many relationships get an "association entity" between them, nothing new.
We write down on paper every SQL statement to build any table, entity or not. We write down every possible primary key or foreign key. The constraint that are not natively satisfied by SQL and/or foreign keys become triggers, and so on.

This exam is considered the true nightmare at our department. I just love it.

Now my question is, do actually DB designers follow this workflow? Or is this just a bloody hard training in Pai Mei style?

I was stuck at this error for the 4-6 days.. Did lots of research on stackoverflow, Google, YT.. Asked my peers tried like hell. Finally one of friends told me you aren't giving I/p and how can you expect an o/p there is no error neither in the compiler nor in the code..
Me: ;_;