[jeffdavis]

> As soon as you start having constraints such as "from date must be before to date" and "the quote must have at least one line, and these lines should sum to > 0 and < 1000000" then relational modelling fails. Hard.

Those types of requirements rarely have a big impact on the design. Sure, you might have to write some procedural code in a trigger, but that's just a handful of lines of code. (I don't know why you think the "from_date < to_date" is a difficult requirement though -- as someone else pointed out, that's just a CHECK constraint).

Relational modelling is often a very clean, concise, and readable way to represent many kinds of businesses. You can pile a few extra requirements on top, and a good DBMS will make it easy to do so.

So I don't see a "hard failure" of anything here.