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Three-Valued Logic and NULL Handling in SQL

SQL's treatment of NULL as unknown, which turns Boolean logic into a three-valued logic of true, false, and unknown. Predicates involving NULL can yield unknown rather than true or false, so different engines' handling of NULL is a frequent source of semantic disagreement.

Questions this Concept answers

Why does `WHERE price = NULL` never match a row?

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NULL and Three-Valued Logic in SQL and Why Comparisons Silently Drop Rows

SQL's NULL denotes *unknown* — not blank, not zero, not empty — and consequently SQL evaluates predicates in a three-valued logic over {TRUE, FALSE, UNKNOWN} rather than Boolean two-valued logic. Any…