Start here. This is the direct spoken answer to practice first.
Why this question matters
Explain database constraints versus application validation with practical .NET data-access examples and production trade-offs. It matters in real backend work because query shape, indexes, transactions, locking, and data volume can turn harmless-looking code into a production bottleneck. The practical angle is database constraints versus application validation, EF Core/SQL behavior, API boundaries, production risks, and practical debugging or design judgment, tied to a concrete production decision.
Application validation gives users early, specific feedback, while database constraints protect truth under concurrency and from every writer. I validate required values, formats, ranges, and known business rules in the application, then use NOT NULL, foreign keys, unique constraints, and check constraints for invariants the database can enforce. The two layers complement each other rather than compete.