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title

SQL Union

🎯 한 줄 통찰 (One-line insight)

UNION combines the result-sets of two or more SELECT statements into one, automatically removing duplicate rows, provided every SELECT has the same number/type/order of columns. [S1]

🧠 핵심 개념 (Core concepts)

UNION operator — combines the result-set of two or more SELECT statements, removing duplicates automatically. [S1]
Requirements — every SELECT must have the same number of columns, similar data types, and columns in the same order. [S1]
Syntax — SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2;. [S1]
Result column naming — result-set column names default to the first SELECT statement's column names. [S1]

🧩 추출된 패턴 (Extracted patterns)

Literal tag column — a literal string like 'Customer' AS Type in one SELECT and 'Supplier' in the other lets a combined UNION query tag which source table each row came from. [S1]
WHERE-filtered UNION — each SELECT in a UNION can carry its own WHERE clause before being combined. [S1]

📖 세부 내용 (Details)

Combine and dedupe countries from two tables: SELECT Country FROM Customers UNION SELECT Country FROM Suppliers ORDER BY Country;. [S1]
Filtered UNION: SELECT City, Country FROM Customers WHERE Country='Germany' UNION SELECT City, Country FROM Suppliers WHERE Country='Germany' ORDER BY City;. [S1]
Tagged UNION distinguishing source: SELECT 'Customer' AS Type, ContactName, City, Country FROM Customers UNION SELECT 'Supplier', ContactName, City, Country FROM Suppliers;. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

소스에서 모순되는 정보는 발견되지 않음. 중복 제거를 원치 않으면 UNION ALL을 쓰라고 명시적으로 안내함. [S1]

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — 이후 UNION ALL 챕터에서 중복 제거 여부만 다른 대응 연산자로 확장된다. [S1]

💻 코드 패턴 (Code patterns)

Combine and dedupe two result-sets (SQL):

SELECT Country FROM Customers
UNION
SELECT Country FROM Suppliers
ORDER BY Country;

✅ 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 Union All, SQL Select Distinct, SQL Order By
참조 맥락: 여러 SELECT 결과를 하나로 합쳐야 할 때의 기본 도구 — 중복 제거가 필요 없다면 UNION ALL을 대신 사용.

📚 출처 (Sources)

[S1] W3Schools — SQL UNION Operator — https://www.w3schools.com/sql/sql_union.asp

📝 변경 이력 (Change history)

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

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