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10_Wiki/Topic_Programming/Topic_SQL/SQL_Group_By.md

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+---
+id: sql-group-by
+title: "SQL Group By"
+category: "Database"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["SQL GROUP BY Statement", "GROUP BY clause", "SQL 그룹화"]
+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", "groupby", "aggregate"]
+raw_sources: ["https://www.w3schools.com/sql/sql_groupby.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[SQL Group By]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+GROUP BY collapses rows sharing the same value into summary rows, and is almost always paired with an aggregate function like COUNT/SUM/AVG to compute a value per group. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **GROUP BY statement** — groups rows with the same values into summary rows, e.g. "find the number of customers in each country". [S1]
+- **Pairs with aggregate functions** — COUNT(), MAX(), MIN(), SUM(), AVG() compute a single value per group. [S1]
+- **Syntax** — `SELECT column1, aggregate_function(column2), column3, ... FROM table_name WHERE condition GROUP BY column1, column3 ORDER BY column_name;`. [S1]
+- **GROUP BY with JOIN** — grouping works across joined tables, e.g. counting orders per shipper after a LEFT JOIN. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **Group-then-sort-by-aggregate** — combining `GROUP BY` with `ORDER BY COUNT(...) DESC` ranks groups by their aggregate value, a very common "top categories" reporting pattern. [S1]
+
+## 📖 세부 내용 (Details)
+- Count customers per country: `SELECT Country, COUNT(CustomerID) AS [Number of Customers] FROM Customers GROUP BY Country;`. [S1]
+- Same, sorted by count descending: `... GROUP BY Country ORDER BY COUNT(CustomerID) DESC;`. [S1]
+- GROUP BY with a JOIN: `SELECT Shippers.ShipperName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders LEFT JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID GROUP BY ShipperName;`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+소스에서 모순되는 정보는 발견되지 않음.
+
+## 🛠️ 적용 사례 (Applied in summary)
+현재 발견된 실제 적용 사례가 없습니다 — 다음 챕터인 HAVING이 GROUP BY 결과 자체를 필터링하는 방법을 확장한다. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+Count rows per group, sorted by count (SQL):
+```sql
+SELECT Country, COUNT(CustomerID) AS [Number of Customers]
+FROM Customers
+GROUP BY Country
+ORDER BY COUNT(CustomerID) DESC;
+```
+
+## ✅ 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 Having]], [[SQL Aggregate Functions]], [[SQL Count]], [[SQL Order By]]
+- **참조 맥락:** 집계 리포트(카테고리별 집계, 랭킹) 작성의 핵심 문법 — HAVING과 함께 이해해야 완전해진다.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — SQL GROUP BY Statement — https://www.w3schools.com/sql/sql_groupby.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "SQL GROUP BY Statement" page (Astra wiki-curation, P-Reinforce v3.1 format).