---
id: sql-unique
title: "SQL Unique"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL UNIQUE Constraint", "UNIQUE 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", "unique", "constraints"]
raw_sources: ["https://www.w3schools.com/sql/sql_unique.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Unique]]

## 🎯 한 줄 통찰 (One-line insight)
UNIQUE guarantees every value in a column is distinct — like PRIMARY KEY, but a table can have many UNIQUE constraints versus only one PRIMARY KEY. [S1]

## 🧠 핵심 개념 (Core concepts)
- **UNIQUE constraint** — ensures all values in a column are unique. [S1]
- **UNIQUE vs. PRIMARY KEY** — both guarantee uniqueness, but a table can have many UNIQUE constraints, and only one PRIMARY KEY. [S1]
- **On CREATE TABLE** — inline (`ID int NOT NULL UNIQUE`) for SQL Server/Oracle/Access, or a separate `UNIQUE (ID)` clause for MySQL. [S1]
- **Named/multi-column UNIQUE** — `CONSTRAINT UC_Person UNIQUE (ID, LastName)` names the constraint and spans multiple columns. [S1]
- **On ALTER TABLE** — `ALTER TABLE Persons ADD UNIQUE (ID);` for an existing table. [S1]
- **Dropping** — `DROP INDEX UC_Person;` (MySQL) vs. `DROP CONSTRAINT UC_Person;` (SQL Server/Oracle/Access). [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **MySQL 문법 차이**: 다른 벤더는 컬럼 옆에 인라인 `UNIQUE` 키워드를 쓰지만, MySQL은 별도의 `UNIQUE (ID)` 절이 필요함. 드롭 시에도 MySQL은 `DROP INDEX`, 나머지는 `DROP CONSTRAINT`로 갈린다. [S1]

## 📖 세부 내용 (Details)
- SQL Server/Oracle/Access inline: `CREATE TABLE Persons (ID int NOT NULL UNIQUE, ...);`. [S1]
- MySQL: `CREATE TABLE Persons (ID int NOT NULL, ..., UNIQUE (ID));`. [S1]
- Named multi-column constraint: `CONSTRAINT UC_Person UNIQUE (ID,LastName)`. [S1]
- Add to existing table: `ALTER TABLE Persons ADD UNIQUE (ID);`. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — PRIMARY KEY가 이미 있는 컬럼 외에 "고유해야 하지만 대표키는 아닌" 컬럼(예: 이메일, 주민번호)에 흔히 쓰인다. [S1]

## 💻 코드 패턴 (Code patterns)
Named multi-column UNIQUE (SQL):
```sql
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT UC_Person UNIQUE (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 Constraints]], [[SQL Primary Key]], [[SQL Not Null]]
- **참조 맥락:** 대표키는 아니지만 값의 고유성을 보장해야 하는 컬럼에 사용.

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

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