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74 lines
3.5 KiB
Markdown
74 lines
3.5 KiB
Markdown
---
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id: sql-union
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title: "SQL Union"
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category: "Database"
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status: "draft"
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verification_status: "conceptual"
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canonical_id: ""
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aliases: ["SQL UNION Operator", "UNION operator", "SQL 합집합"]
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duplicate_of: ""
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source_trust_level: "B"
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confidence_score: 0.9
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created_at: 2026-07-04
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updated_at: 2026-07-04
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review_reason: ""
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merge_history: []
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tags: ["sql", "database", "w3schools", "union"]
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raw_sources: ["https://www.w3schools.com/sql/sql_union.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[SQL Union]]
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## 🎯 한 줄 통찰 (One-line insight)
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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]
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## 🧠 핵심 개념 (Core concepts)
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- **UNION operator** — combines the result-set of two or more SELECT statements, removing duplicates automatically. [S1]
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- **Requirements** — every SELECT must have the same number of columns, similar data types, and columns in the same order. [S1]
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- **Syntax** — `SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2;`. [S1]
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- **Result column naming** — result-set column names default to the first SELECT statement's column names. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **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]
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- **WHERE-filtered UNION** — each SELECT in a UNION can carry its own WHERE clause before being combined. [S1]
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## 📖 세부 내용 (Details)
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- Combine and dedupe countries from two tables: `SELECT Country FROM Customers UNION SELECT Country FROM Suppliers ORDER BY Country;`. [S1]
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- Filtered UNION: `SELECT City, Country FROM Customers WHERE Country='Germany' UNION SELECT City, Country FROM Suppliers WHERE Country='Germany' ORDER BY City;`. [S1]
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- Tagged UNION distinguishing source: `SELECT 'Customer' AS Type, ContactName, City, Country FROM Customers UNION SELECT 'Supplier', ContactName, City, Country FROM Suppliers;`. [S1]
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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소스에서 모순되는 정보는 발견되지 않음. 중복 제거를 원치 않으면 UNION ALL을 쓰라고 명시적으로 안내함. [S1]
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## 🛠️ 적용 사례 (Applied in summary)
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현재 발견된 실제 적용 사례가 없습니다 — 이후 UNION ALL 챕터에서 중복 제거 여부만 다른 대응 연산자로 확장된다. [S1]
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## 💻 코드 패턴 (Code patterns)
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Combine and dedupe two result-sets (SQL):
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```sql
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SELECT Country FROM Customers
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UNION
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SELECT Country FROM Suppliers
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ORDER BY Country;
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```
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## ✅ 검증 상태 및 신뢰도
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- **상태:** draft
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- **검증 단계:** conceptual
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- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
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- **신뢰 점수:** 0.90
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- **중복 검사 결과:** 신규 생성 (New discovery)
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## 🔗 지식 그래프 (Knowledge Graph)
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- **상위/루트:** [[SQL Tutorial]]
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- **관련 개념:** [[SQL Union All]], [[SQL Select Distinct]], [[SQL Order By]]
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- **참조 맥락:** 여러 SELECT 결과를 하나로 합쳐야 할 때의 기본 도구 — 중복 제거가 필요 없다면 UNION ALL을 대신 사용.
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## 📚 출처 (Sources)
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- [S1] W3Schools — SQL UNION Operator — https://www.w3schools.com/sql/sql_union.asp
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## 📝 변경 이력 (Change history)
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- 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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