Keys are what connect tables.
Relational database systems got along for almost 20 years quite well
without the PRIMARY and FOREIGN key declarations. MySQL
is an example of a popular database that ignored primary and foreign
key declarations until recent versions.
So why were they added to SQL2, and what can they enable a database to
that can't be done without them?
- They provide useful documentation to humans.
- They allow QBE systems to automatically generate queries.
- They allow enforcing referential integrity.