"Remove Duplicates" Requires a Definition

Concepts covered: sqlWindowDedup

Two kinds of duplicate: exact vs by key An exact duplicate is a row where every column has the same value as another row. Two rows with the same customer_id, same order_date, same amount, same created_at: byte-for-byte identical. DISTINCT removes these cleanly. A duplicate by key is a row where some subset of columns has the same value but other columns differ. Two rows with the same customer_id and order_date, but different created_at and different amount (because one is a correction). DISTINCT cannot help here because the rows are not identical; you have to pick which copy to keep based on a business rule. Almost every interview deduplication question is the second case. Why companies care Deduplication powers every data-platform invariant. CDC streams (the same row gets emitted multiple

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