When I was learning in university, they taught us the database fundamentals, basics and rules, and one of the most important rules is the constraints (primary key, foreign key), and how to make 1-m, 1-1, m-n relationships.
Now when I move to real business environment they tell me: you should forget all you have been taught; no constraints, all those relationships are logical, no primary keys, no foreign keys, you can make your constraints through the code.
I don't know who is right: what I learned in my academic life or what I will learn in my new real business life. What do you think?
I think the constraints help you to have clean data. Performance is sometimes improved. In some cases, the performance can get affected by having the constraints. However, the answer to that is not removing the constraints. You have something called "denormalization" to help you deal with the performance issues (provided that your queries are already optimized). You can always create denormalized summary tables in such scenarios.
Did the guys who told you to "forget what you learnt" also tell you that they have forgotten the traffic rules they learnt at the driving classes?