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title

SQL Primary Key

🎯 한 줄 통찰 (One-line insight)

PRIMARY KEY uniquely identifies every row (as a UNIQUE + NOT NULL combination), can span multiple columns, and is the anchor that FOREIGN KEY constraints in other tables point to. [S1]

🧠 핵심 개념 (Core concepts)

PRIMARY KEY constraint — uniquely identifies each record; ensures unique values and cannot contain NULL (combination of UNIQUE + NOT NULL). [S1]
One per table — a table can have only ONE PRIMARY KEY, which can be a single column or a combination of columns (composite key). [S1]
FK anchor — the primary key is the target that FOREIGN KEY constraints in other tables reference, enforcing referential integrity. [S1]
ALTER TABLE prerequisite — adding a PRIMARY KEY via ALTER TABLE requires the column(s) already be declared NOT NULL. [S1]

📖 세부 내용 (Details)

Single-column PK on creation: CREATE TABLE Persons (ID int PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int);. [S1]
Composite (multi-column) PK: PRIMARY KEY (ID, LastName), or named: CONSTRAINT PK_Person PRIMARY KEY (ID, LastName). [S1]
Add PK to existing table: ALTER TABLE Persons ADD PRIMARY KEY (ID);. [S1]
Drop PK: ALTER TABLE Persons DROP CONSTRAINT PK_Person; (SQL Server/Oracle/Access) vs. ALTER TABLE Persons DROP PRIMARY KEY; (MySQL). [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

드롭 문법 차이: SQL Server/Oracle/Access는 DROP CONSTRAINT, MySQL은 DROP PRIMARY KEY를 사용. [S1]

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — FOREIGN KEY 챕터에서 이 PRIMARY KEY를 참조하는 관계가 바로 이어서 다뤄진다. [S1]

💻 코드 패턴 (Code patterns)

Composite primary key (SQL):

CREATE TABLE Persons (
    ID int,
    LastName varchar(255),
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID, LastName)
);

✅ 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 Foreign Key, SQL Not Null, SQL Unique, SQL Auto Increment
참조 맥락: 테이블 설계의 핵심 — FOREIGN KEY가 참조하는 대상이자, 대개 AUTO INCREMENT와 함께 쓰인다.

📚 출처 (Sources)

[S1] W3Schools — SQL PRIMARY KEY Constraint — https://www.w3schools.com/sql/sql_primarykey.asp

📝 변경 이력 (Change history)

2026-07-04: Initial draft synthesized from the W3Schools "SQL PRIMARY KEY Constraint" page (Astra wiki-curation, P-Reinforce v3.1 format).

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