---
id: sql-create-index
title: "SQL Create Index"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL CREATE INDEX Statement", "CREATE INDEX", "CREATE UNIQUE INDEX"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.88
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["sql", "database", "w3schools", "index"]
raw_sources: ["https://www.w3schools.com/sql/sql_create_index.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Create Index]]

## 🎯 한 줄 통찰 (One-line insight)
Indexes speed up data retrieval on frequently-searched columns, but slow down writes since the index itself must be updated — a classic read/write trade-off, not a free performance win. [S1]

## 🧠 핵심 개념 (Core concepts)
- **CREATE INDEX statement** — creates indexes on table columns to speed up data retrieval; invisible to users, purely a performance aid. [S1]
- **Write cost trade-off** — updating indexed tables is more time-consuming, since indexes must also be updated — so index only columns that are frequently searched. [S1]
- **Non-unique vs. unique** — `CREATE INDEX` allows duplicate values; `CREATE UNIQUE INDEX` does not. [S1]
- **Multi-column index** — listing multiple columns creates a composite index. [S1]
- **DROP INDEX** — vendor-specific syntax to remove an index. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **DROP INDEX 문법이 4가지 벤더 모두 다름**: SQL Server(`DROP INDEX table.index`), MySQL(`ALTER TABLE ... DROP INDEX`), MS Access(`DROP INDEX index ON table`), DB2/Oracle(`DROP INDEX index`). [S1]

## 📖 세부 내용 (Details)
- Non-unique index: `CREATE INDEX idx_lastname ON Persons (LastName);`. [S1]
- Composite index: `CREATE INDEX idx_lname_fname ON Persons (LastName, FirstName);`. [S1]
- Unique index syntax: `CREATE UNIQUE INDEX index_name ON table_name (column1, column2, ...);`. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 자주 검색되는 컬럼(예: LastName)에만 선택적으로 적용해야 쓰기 성능 저하를 최소화할 수 있다. [S1]

## 💻 코드 패턴 (Code patterns)
Single and composite index (SQL):
```sql
CREATE INDEX idx_lastname
ON Persons (LastName);

CREATE INDEX idx_lname_fname
ON Persons (LastName, FirstName);
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[SQL Tutorial]]
- **관련 개념:** [[SQL Constraints]], [[SQL Unique]], [[SQL Where]]
- **참조 맥락:** 자주 검색되는 컬럼의 조회 성능을 높일 때 사용 — 쓰기 성능 저하와 트레이드오프임에 유의.

## 📚 출처 (Sources)
- [S1] W3Schools — SQL CREATE INDEX Statement — https://www.w3schools.com/sql/sql_create_index.asp

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