2nd/10_Wiki/Dev/Topic_SQL/SQL_Inner_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-inner-join	SQL Inner Join	Database	draft	conceptual		INNER JOIN SQL 내부 조인		B	0.9	2026-07-04	2026-07-04			sql database w3schools join inner-join	https://www.w3schools.com/sql/sql_join_inner.asp

SQL Inner Join

🎯 한 줄 통찰 (One-line insight)

INNER JOIN — the default join type, so plain JOIN means the same thing — returns only rows that have a match in both tables, discarding anything unmatched. [S1]

🧠 핵심 개념 (Core concepts)

• INNER JOIN — returns only rows that have matching values in both tables. [S1]
• JOIN == INNER JOIN — JOIN alone defaults to INNER JOIN; the parser treats them identically. [S1]
• Syntax — SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name;. [S1]
• Multi-table INNER JOIN — more than two tables can be joined by chaining multiple INNER JOIN clauses. [S1]

🧩 추출된 패턴 (Extracted patterns)

• Table-name qualification is a good habit — prefixing column names with their table (Products.ProductID) avoids ambiguity errors when the same column name (e.g. CategoryID) exists in both joined tables. [S1]
• Unmatched rows silently vanish — a product with no CategoryID, or with a CategoryID absent from the Categories table, is simply excluded from the result — a common footgun when INNER JOIN is used where LEFT JOIN was intended. [S1]

📖 세부 내용 (Details)

• Basic two-table join: SELECT ProductID, ProductName, CategoryName FROM Products INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;. [S1]
• Qualified column names to avoid ambiguity: SELECT Products.ProductID, Products.ProductName, Categories.CategoryName FROM Products INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;. [S1]
• JOIN is shorthand for INNER JOIN — both produce identical results. [S1]
• Joining three tables: FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID INNER JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID;. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

소스에서 모순되는 정보는 발견되지 않음.

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — SQL Aliases 챕터의 테이블 별칭(c, o)이 INNER JOIN 예제에서도 그대로 재사용된다. [S1]

💻 코드 패턴 (Code patterns)

Two-table INNER JOIN (SQL):

SELECT Products.ProductID, Products.ProductName, Categories.CategoryName
FROM Products
INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;

Three-table INNER JOIN chain (SQL):

SELECT Orders.OrderID, Customers.CustomerName, Shippers.ShipperName
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID
INNER JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID;

✅ 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 Joins, SQL Left Join, SQL Aliases
• 참조 맥락: 매칭되는 행만 필요한 조인 쿼리의 기본 유형 — LEFT/RIGHT/FULL JOIN과 대비해서 이해해야 한다.

📚 출처 (Sources)

• [S1] W3Schools — SQL INNER JOIN — https://www.w3schools.com/sql/sql_join_inner.asp

📝 변경 이력 (Change history)

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