Could you please explain me why it is necessary to specify those foreign keys when creating tables? I mean, I've created two tables which have a one-to-many relationship (on the ER-diagram) but I didn't specify the foreign keys and references. I can connect the tables using the where-clause and even perform joins and so on.
Probably, I don't get some basic concepts, although I've read some stuff about it. I guess it has something to do with data consistency or referential integrity or something.
So, could you explain me these concepts? Are those references and foreign keys are absolutely required if I have, let's say, 8-10 tables with one-to-many relationships and if I can assure that the data is inserted correctly into the database?