Treat NULL as unknown, not zero

NULL makes SQL more honest and more dangerous. It preserves the difference between zero and unknown, but it also means ordinary comparison logic is not enough. A filter keeps true, drops false, and also drops unknown. Joins on nullable keys may fail to match rows that look similar. Aggregates vary: COUNT(*) counts rows, COUNT(column) counts known values. Professionals do not hide NULL by reflex; they decide whether unknown should be excluded, labeled, or transformed.

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