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10_Wiki/Topic_Programming/Topic_SQL/SQL_Union.md

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+---
+id: sql-union
+title: "SQL Union"
+category: "Database"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["SQL UNION Operator", "UNION operator", "SQL 합집합"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.9
+created_at: 2026-07-04
+updated_at: 2026-07-04
+review_reason: ""
+merge_history: []
+tags: ["sql", "database", "w3schools", "union"]
+raw_sources: ["https://www.w3schools.com/sql/sql_union.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[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):
+```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).