Constraints & Keys

Summary

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.

MySQL

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