2nd/10_Wiki/Dev/Topic_SQL/SQL_Null_Values.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
sql-null-values	SQL Null Values	Database	draft	conceptual		NULL values IS NULL IS NOT NULL SQL 널 값		B	0.9	2026-07-04	2026-07-04			sql database w3schools null	https://www.w3schools.com/sql/sql_null_values.asp

SQL Null Values

🎯 한 줄 통찰 (One-line insight)

NULL represents an unknown or absent value — distinct from zero or an empty string — and can only be tested with IS NULL / IS NOT NULL, never with =, <, or <>. [S1]

🧠 핵심 개념 (Core concepts)

• NULL value — represents unknown, missing, or inapplicable data; a placeholder for absence of data, not a value itself. [S1]
• NULL ≠ 0 or '' — a NULL field is one left blank at record creation, distinct from zero or an empty string. [S1]
• Comparison operators don't work on NULL — =, <, <> cannot test for NULL; use IS NULL / IS NOT NULL instead. [S1]
• Syntax — SELECT column_names FROM table_name WHERE column_name IS NULL; and the IS NOT NULL counterpart. [S1]

🧩 추출된 패턴 (Extracted patterns)

• Dedicated NULL-test operators — because ordinary comparison operators silently fail to match NULL, SQL provides IS NULL/IS NOT NULL as the only reliable test — a common footgun for beginners who try WHERE column = NULL. [S1]

📖 세부 내용 (Details)

• Find rows with a NULL field: SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL;. [S1]
• Find rows with a non-NULL field: SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NOT NULL;. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

소스에서 모순되는 정보는 발견되지 않음.

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — LEFT JOIN의 anti-join 패턴(WHERE Orders.CustomerID IS NULL)에서 이미 이 문법이 선행 사용되었다. [S1]

💻 코드 패턴 (Code patterns)

Test for NULL / NOT NULL (SQL):

SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address IS NULL;

✅ 검증 상태 및 신뢰도

• 상태: draft
• 검증 단계: conceptual
• 출처 신뢰도: B (W3Schools — widely used educational reference, not a primary standards body)
• 신뢰 점수: 0.90
• 중복 검사 결과: 신규 생성 (New discovery)

🔗 지식 그래프 (Knowledge Graph)

• 상위/루트: SQL Tutorial
• 관련 개념: SQL Where, SQL Left Join, SQL Null Functions
• 참조 맥락: NULL을 다뤄야 하는 모든 쿼리의 기초 — 이후 SQL Null Functions(COALESCE 등)에서 NULL 대체 함수로 확장.

📚 출처 (Sources)

• [S1] W3Schools — SQL NULL Values — https://www.w3schools.com/sql/sql_null_values.asp

📝 변경 이력 (Change history)

• 2026-07-04: Initial draft synthesized from the W3Schools "SQL NULL Values" page (Astra wiki-curation, P-Reinforce v3.1 format).