---
id: sql-self-join
title: "SQL Self Join"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["Self Join", "SQL 자기 조인"]
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", "join", "self-join"]
raw_sources: ["https://www.w3schools.com/sql/sql_join_self.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Self Join]]

## 🎯 한 줄 통찰 (One-line insight)
A self join is a regular join where a table is joined with itself, made possible by giving the same table two different aliases so rows can be compared against other rows in the same table. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Self join** — a regular join where the table is joined with itself. [S1]
- **Syntax** — `SELECT column_name(s) FROM table1 T1, table1 T2 WHERE condition;` where T1 and T2 are different aliases for the same table. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Exclude self-match** — a self join typically needs a condition like `A.CustomerID <> B.CustomerID` to avoid matching each row with itself. [S1]
- **Dual-alias comparison** — the same table appears twice with different aliases (A, B) purely so SQL can treat the two references as independent row sets for comparison. [S1]

## 📖 세부 내용 (Details)
- Match customers that share the same city (excluding matching a customer with itself): [S1]
```sql
SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City
FROM Customers A, Customers B
WHERE A.CustomerID <> B.CustomerID
AND A.City = B.City
ORDER BY A.City;
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
소스에서 모순되는 정보는 발견되지 않음.

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 조직도(상사-부하 관계), 동일 도시 고객 찾기 등 "같은 테이블 내 행 간 관계"를 표현할 때 쓰이는 전형적 패턴이다. [S1]

## 💻 코드 패턴 (Code patterns)
Self join with alias and self-match exclusion (SQL):
```sql
SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City
FROM Customers A, Customers B
WHERE A.CustomerID <> B.CustomerID
AND A.City = B.City
ORDER BY A.City;
```

## ✅ 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 Joins]], [[SQL Aliases]], [[SQL Inner Join]]
- **참조 맥락:** 같은 테이블 내 행 간 관계를 표현해야 할 때(계층 구조, 동일 그룹 찾기) 재사용.

## 📚 출처 (Sources)
- [S1] W3Schools — SQL Self Join — https://www.w3schools.com/sql/sql_join_self.asp

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