2nd/10_Wiki/Dev/Topic_SQL/SQL_Self_Join.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
sql-self-join	SQL Self Join	Database	draft	conceptual		Self Join SQL 자기 조인		B	0.88	2026-07-04	2026-07-04			sql database w3schools join self-join	https://www.w3schools.com/sql/sql_join_self.asp

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]

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):

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).