---
id: sql-check
title: "SQL Check"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL CHECK Constraint", "CHECK 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", "check", "constraints"]
raw_sources: ["https://www.w3schools.com/sql/sql_check.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Check]]

## 🎯 한 줄 통찰 (One-line insight)
CHECK enforces an arbitrary boolean condition on a column (or combination of columns) — an INSERT/UPDATE that would evaluate the condition to FALSE is aborted entirely. [S1]

## 🧠 핵심 개념 (Core concepts)
- **CHECK constraint** — ensures values in a column satisfy a specific condition; evaluates to TRUE or FALSE, and FALSE aborts the entire INSERT/UPDATE with an error. [S1]
- **On CREATE TABLE** — inline, e.g. `Age int CHECK (Age >= 18)`. [S1]
- **Named, multi-column CHECK** — `CONSTRAINT chk_PersonAge CHECK (Age >= 18 AND City = 'Sandnes')` combines multiple column conditions under one name. [S1]
- **On ALTER TABLE** — `ALTER TABLE Persons ADD CHECK (Age >= 18);` or the named/multi-column form via `ADD CONSTRAINT`. [S1]
- **Dropping** — `DROP CONSTRAINT chk_PersonAge;` (SQL Server/Oracle/Access) vs. `DROP CHECK chk_PersonAge;` (MySQL). [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **드롭 문법 차이**: SQL Server/Oracle/Access는 `DROP CONSTRAINT`, MySQL은 `DROP CHECK`를 사용. [S1]

## 📖 세부 내용 (Details)
- Single-column CHECK: `CREATE TABLE Persons (ID int PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int CHECK (Age >= 18));`. [S1]
- Named, multi-column CHECK: `CONSTRAINT chk_PersonAge CHECK (Age >= 18 AND City = 'Sandnes')`. [S1]
- Add to existing table: `ALTER TABLE Persons ADD CHECK (Age >= 18);`. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — DEFAULT/NOT NULL만으로 표현할 수 없는 임의 비즈니스 규칙(나이 제한, 값 범위)을 강제할 때 쓰인다. [S1]

## 💻 코드 패턴 (Code patterns)
Named CHECK across two columns (SQL):
```sql
CREATE TABLE Persons (
    ID int PRIMARY KEY,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255),
    CONSTRAINT chk_PersonAge CHECK (Age >= 18 AND City = 'Sandnes')
);
```

## ✅ 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 Default]], [[SQL Not Null]]
- **참조 맥락:** 임의의 비즈니스 규칙을 컬럼 값에 강제할 때 사용.

## 📚 출처 (Sources)
- [S1] W3Schools — SQL CHECK Constraint — https://www.w3schools.com/sql/sql_check.asp

## 📝 변경 이력 (Change history)
- 2026-07-04: Initial draft synthesized from the W3Schools "SQL CHECK Constraint" page (Astra wiki-curation, P-Reinforce v3.1 format).