Here is a customer. Here are his / her purchases. Which items would we recommend to him / her? Would we send her / him stuff without upfront payment? Only by credit card? <- that is what I was referring to.

Yes. All very interesting questions for a nice ERD.

I don't see how you can answer that question by looking at an ERD, sorry.

CREATE TABLE customer ( .., invoice_payment BOOLEAN );

I was referring to an algorithm that would take the purchase data and determine whether whether an upfront payment is required.

CREATE VIEW ... (...) 😉

Ah. I didn't know that algorithms implemented in DDL are data.... 😉

I get your point of course. To me, data and logic is all the same. The beauty of the relational model is that it already includes relational algebra to create and operate on derived data. You're doing the same, but you call it "API" and probably imply it is done elsewhere.

I sympathize with your view, @lukaseder; it’s similar to my favorite architectural style in that it tries to cast everything into a consistent model. And the tons of shitty REST service implementations could be compared to the typical unmaintainable PL/SQL catastrophes.

Yes, REST and PL/SQL APIs have much in common. Could the biggest commonalities leading to catastrophe be caused in part by Conway's Law? Perhaps, Conway's Law is also a factor against data centricity, which is probably harder to maintain across organisational boundaries...