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title

SQL Joins

🎯 한 줄 통찰 (One-line insight)

JOIN combines rows from two or more tables based on a related column between them, with four variants (INNER/LEFT/RIGHT/FULL) differing in which unmatched rows they keep. [S1]

🧠 핵심 개념 (Core concepts)

JOIN clause — combines rows from two or more tables based on a related column. [S1]
(INNER) JOIN — returns only rows with matching values in both tables. [S1]
LEFT (OUTER) JOIN — returns all rows from the left table, plus matched rows from the right table. [S1]
RIGHT (OUTER) JOIN — returns all rows from the right table, plus matched rows from the left table. [S1]
FULL (OUTER) JOIN — returns all rows when there is a match in either table. [S1]

🧩 추출된 패턴 (Extracted patterns)

Related-column identification — before writing a JOIN, identify the shared column between two tables (e.g. CustomerID appears in both "Orders" and "Customers"). [S1]

📖 세부 내용 (Details)

Example relationship: the "Orders" table's CustomerID refers to the "Customers" table's CustomerID. [S1]
INNER JOIN combining the two: SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;. [S1]
The four JOIN types are summarized as covering: matches-only (INNER), left-preserving (LEFT), right-preserving (RIGHT), and both-preserving (FULL). [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

소스에서 모순되는 정보는 발견되지 않음.

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — 이 문서는 개론이며, 각 JOIN 유형은 이어지는 개별 챕터(Inner/Left/Right/Full/Self Join)에서 상세히 다뤄진다. [S1]

💻 코드 패턴 (Code patterns)

Basic INNER JOIN (SQL):

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;

✅ 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 Inner Join, SQL Left Join, SQL Right Join, SQL Full Join, SQL Aliases
참조 맥락: JOIN 계열 챕터 전체의 개론 — 이후 4가지 JOIN 유형이 각각 별도 문서로 심화된다.

📚 출처 (Sources)

[S1] W3Schools — SQL Joins — https://www.w3schools.com/sql/sql_join.asp

📝 변경 이력 (Change history)

2026-07-04: Initial draft synthesized from the W3Schools "SQL Joins" page (Astra wiki-curation, P-Reinforce v3.1 format).

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