---
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).