---
id: sql-foreign-key
title: "SQL Foreign Key"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL FOREIGN KEY Constraint", "FOREIGN KEY 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", "foreignkey", "constraints"]
raw_sources: ["https://www.w3schools.com/sql/sql_foreignkey.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Foreign Key]]

## 🎯 한 줄 통찰 (One-line insight)
FOREIGN KEY links a child table's column to a parent table's PRIMARY KEY, blocking both invalid inserts into the child and deletes from the parent that would orphan child rows. [S1]

## 🧠 핵심 개념 (Core concepts)
- **FOREIGN KEY constraint** — establishes a link between two tables and prevents actions that would destroy that link. [S1]
- **Child vs. parent table** — the table holding the foreign key column is the child table; the table holding the referenced primary key is the parent (referenced) table. [S1]
- **Two-way protection** — blocks invalid inserts into the child (value must exist in the parent), and blocks deletes from the parent that would orphan matching child rows. [S1]
- **Syntax (on create)** — `CONSTRAINT fk_Person FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)` inside `CREATE TABLE`. [S1]
- **Syntax (on alter)** — `ALTER TABLE Orders ADD CONSTRAINT fk_Person FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);`. [S1]

## 📖 세부 내용 (Details)
- Define on table creation: [S1]
```sql
CREATE TABLE Orders (
    OrderID int PRIMARY KEY,
    OrderNumber int NOT NULL,
    PersonID int,
    CONSTRAINT fk_Person
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
```
- Add to an existing table: `ALTER TABLE Orders ADD CONSTRAINT fk_Person FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);`. [S1]
- Drop: `ALTER TABLE Orders DROP CONSTRAINT fk_Person;` (SQL Server/Oracle/Access) vs. `ALTER TABLE Orders DROP FOREIGN KEY fk_Person;` (MySQL). [S1]

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

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — DROP TABLE 챕터에서 이미 "FK로 참조되는 테이블은 바로 드롭할 수 없다"는 제약이 선행 언급되었다. [S1]

## 💻 코드 패턴 (Code patterns)
FOREIGN KEY on table creation (SQL):
```sql
CREATE TABLE Orders (
    OrderID int PRIMARY KEY,
    OrderNumber int NOT NULL,
    PersonID int,
    CONSTRAINT fk_Person
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
```

## ✅ 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 Primary Key]], [[SQL Drop Table]], [[SQL Inner Join]]
- **참조 맥락:** 테이블 간 참조 무결성을 강제할 때 사용 — JOIN에서 활용되는 관계가 바로 이 제약으로 정의된다.

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

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