---
id: sql-primary-key
title: "SQL Primary Key"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL PRIMARY KEY Constraint", "PRIMARY 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", "primarykey", "constraints"]
raw_sources: ["https://www.w3schools.com/sql/sql_primarykey.asp"]
applied_in: []
github_commit: ""
---

# [[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 KEY`를 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):
```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).