NULL in SQL: three-valued logic and its traps

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article · en · 지식 기준일 2026-09-15 · 변경일 2026-09-23, 리비전 2 · reviewed (검토 기록됨 2026-09-23)

NULL means unknown, so comparisons with NULL yield unknown rather than true or false; WHERE filters drop unknown rows, NOT IN with a NULL matches nothing, and aggregates skip NULLs. Use IS NULL, IS DISTINCT FROM and COALESCE deliberately.

What it is

SQL uses three truth values: true, false and unknown. Any comparison with NULL (x = NULL, x <> NULL, x > 5 when x is NULL) yields unknown. WHERE keeps only rows whose condition is true, so unknown behaves like false there, but NOT (unknown) is still unknown. The PostgreSQL documentation describes IS NULL, IS NOT NULL and IS [NOT] DISTINCT FROM, which treat NULL as a comparable value.

Why it matters

The classic bugs: WHERE status <> 'archived' silently excludes rows with NULL status; id NOT IN (SELECT parent_id FROM t) returns no rows if any parent_id is NULL; COUNT(col) counts non-NULL values while COUNT(*) counts rows; AVG ignores NULLs; a unique constraint allows several NULLs (unless NULLS NOT DISTINCT is specified).

How to apply

Decide per column whether NULL is allowed and what it means; add NOT NULL wherever there is no meaning for "unknown".
Use IS DISTINCT FROM for equality that treats NULL as a value; use COALESCE to give NULL an explicit default in expressions.
Prefer NOT EXISTS over NOT IN with subqueries that may contain NULLs.
In application code, check how the driver maps NULL (None, nil, null) and keep it distinct from empty strings and zero.

Pitfalls

Sorting: NULLs sort first or last depending on the database and NULLS FIRST/LAST. String concatenation with NULL yields NULL in standard SQL. GROUP BY puts all NULLs in one group even though NULL is not equal to NULL.

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지식 기준일: 2026-09-15. 상태: reviewed — 편집하면 검토 상태가 초기화됩니다. 본문은 검증되지 않은 참고 자료로 다루고 출처를 확인하세요.

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편집자 계정 344519e7-8ea1-44c6-abaa-29102abda2b6가 2026-09-23에 리비전 2을 검토한 기록입니다. 현재 리비전에 적용: 예.

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

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