Greg Low violates Betteridge’s Law of Headlines:
Foreign keys are used to ensure referential integrity in relational databases. We don’t want to have orders for customers that don’t exist, or have lines on the orders that refer to products that don’t exist. That seems straightforward enough – and a good idea – so why is there an endless discussion in the development community about whether databases should include declared foreign key constraints or not?
I tend to be a foreign key extremist, in that I want key constraints whenever possible. There are certain specific cases in which foreign key constraints can’t work, but I’d argue they’re much rarer than people think.