This is probably a trivial question, but I'm still a little clumsy when it comes to foreign key constraints so I wanted to make sure.
Let's say I have a table countries with the fields country_id (PK) and name, and a table cities with the fields city_id (PK), name and country_id (FK).
The foreign key cities.country_id has the constraint ON DELETE SET NULL. As I understand it, this means that if a record from countries is deleted, any records in cities that reference that deleted record's country_id will have its country_id field set to NULL.
What if, however, cities.country_id has the attribute NOT NULL? Will this prevent the foreign key constraint from working properly? It would make sense that it does, but I just want to check.