docs(10_Wiki): 위키 구조 정리 — 언어 튜토리얼 카테고리 폴더 제거 + 신규 자산 동기화

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