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title

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]

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 KEY를 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):

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).

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