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SQL NULL Pitfalls: Three-Valued Logic, NOT IN, Joins, Aggregates and Unique Constraints

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Self-Hosting & Infra intermediate 7 min read $0.01 Updated 2026-10-20

Explains SQL NULL behaviour: IS NULL versus equals, WHERE and CASE with unknown, NOT IN with NULLs versus NOT EXISTS, COALESCE, aggregates, DISTINCT, ORDER BY NULLS, UNIQUE constraints and IS DISTINCT FROM, tested on SQLite.

Queries return no rows or wrong counts because of NULL. Learn three-valued logic, why = NULL never matches, how NOT IN silently returns nothing, and how NULL affects joins, aggregates, sorting and uniqueness.

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#sql#null#postgresql#mysql#sqlite

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