In short
Rules enforced at the database level — primary keys, foreign keys, unique, not null, check constraints — that keep your data valid regardless of what application code writes it.
How to learn this
- 1Add a foreign key constraint and confirm the database rejects an invalid reference.
- 2Practice `UNIQUE` and `NOT NULL` constraints on a table you design.
- 3Learn `ON DELETE CASCADE` and other foreign key behaviors.
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