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Incoming links: Subquery Expressions in the PostgreSQL documentation
SQL NULL Created 2026-10-31 Updated 2026-11-05
SQL NULL marks a missing or unknown value. Ordinary comparisons involving SQL NULL, including
NULL = NULL, produce the truth value unknown. To test a scalar value for absence, use IS NULL; to compare while treating two missing values alike, use IS NOT DISTINCT FROM.[ref]Comparison Functions and Operators in the PostgreSQL documentation. version 18, section 9.2, null comparisons and predicates
This three-valued logic changes query reasoning.
5 NOT IN (2, NULL) is unknown: five differs from two, but the missing value might be five. A relational selection retains true conditions, so unknown does not pass the filter.[ref]Subquery Expressions in the PostgreSQL documentation. version 18, section 9.24.3, NOT IN with null inputs
Database aggregation has operation-specific rules. On values
2, NULL, 4, COUNT(*) counts three rows, COUNT(value) counts two non-null values and AVG(value) is three. With no input rows, COUNT returns zero while AVG and SUM return SQL NULL. Therefore a blanket rule that all database aggregation ignores SQL NULL is insufficient.[ref]Aggregate Functions in the PostgreSQL documentation. version 18, section 9.21, count variants and empty-input behavior