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10_Wiki/Dev/Topic_SQL/SQL_Not_Null.md

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+---
+id: sql-not-null
+title: "SQL Not Null"
+category: "Database"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["SQL NOT NULL Constraint", "NOT NULL constraint"]
+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", "notnull", "constraints"]
+raw_sources: ["https://www.w3schools.com/sql/sql_notnull.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[SQL Not Null]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+NOT NULL forces a column to always hold a value — columns are nullable by default — and can be added to or removed from an existing table via vendor-specific ALTER TABLE syntax. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **NOT NULL constraint** — enforces a column to not accept NULL values; you cannot insert/update a record without a value for that field. [S1]
+- **Default is nullable** — columns accept NULL by default unless NOT NULL is specified. [S1]
+- **On CREATE TABLE** — add `NOT NULL` after the column's data type. [S1]
+- **On ALTER TABLE** — vendor-specific: SQL Server/MS Access use `ALTER COLUMN`, MySQL uses `MODIFY COLUMN`, Oracle 10G+ uses `MODIFY`. [S1]
+- **Removing NOT NULL** — same vendor-specific ALTER syntax, but specifying `NULL` instead of `NOT NULL`. [S1]
+
+## 📖 세부 내용 (Details)
+- On table creation: `CREATE TABLE Persons (ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255) NOT NULL, Age int);`. [S1]
+- Add NOT NULL after creation (SQL Server/Access): `ALTER TABLE Persons ALTER COLUMN Age int NOT NULL;`. [S1]
+- Add NOT NULL after creation (MySQL): `ALTER TABLE Persons MODIFY COLUMN Age int NOT NULL;`. [S1]
+- Remove NOT NULL (SQL Server/Access): `ALTER TABLE Persons ALTER COLUMN Age int NULL;`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+- **벤더별 ALTER 문법 차이**: SQL Server/Access(`ALTER COLUMN`) vs. MySQL(`MODIFY COLUMN`) vs. Oracle(`MODIFY`)가 서로 다름. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+현재 발견된 실제 적용 사례가 없습니다 — PRIMARY KEY의 구성 요소(UNIQUE + NOT NULL) 중 하나로 재사용된다. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+NOT NULL on table creation (SQL):
+```sql
+CREATE TABLE Persons (
+    ID int NOT NULL,
+    LastName varchar(255) NOT NULL,
+    FirstName varchar(255) NOT NULL,
+    Age int
+);
+```
+
+## ✅ 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 Constraints]], [[SQL Primary Key]], [[SQL Null Values]]
+- **참조 맥락:** 필수 입력 컬럼을 강제할 때 사용 — PRIMARY KEY 구성 요소 중 하나.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — SQL NOT NULL Constraint — https://www.w3schools.com/sql/sql_notnull.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "SQL NOT NULL Constraint" page (Astra wiki-curation, P-Reinforce v3.1 format).