Primary key uniquely identifies a row and is the main join key (one per table). Unique also forbids duplicates but can allow nulls (DB-dependent). ShopNest Orders.Id is PK; Email can be UNIQUE.
EY rounds often probe: Client scenarios.
Themes commonly reported in public MNC interview experiences (AmbitionBox / LinkedIn / Glassdoor-style). Practice aloud; keep answers this short in the real round.