2nd/10_Wiki/Dev/Topic_SQL/SQL_Group_By.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-group-by	SQL Group By	Database	draft	conceptual		SQL GROUP BY Statement GROUP BY clause SQL 그룹화		B	0.9	2026-07-04	2026-07-04			sql database w3schools groupby aggregate	https://www.w3schools.com/sql/sql_groupby.asp

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):

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).