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10_Wiki/Topic_Programming/Topic_SQL/SQL_Left_Join.md

@@ -0,0 +1,78 @@
+---
+id: sql-left-join
+title: "SQL Left Join"
+category: "Database"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["LEFT JOIN", "LEFT OUTER JOIN", "SQL 왼쪽 외부 조인"]
+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", "join", "left-join"]
+raw_sources: ["https://www.w3schools.com/sql/sql_join_left.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[SQL Left Join]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+LEFT JOIN keeps every row from the left table regardless of a match, filling unmatched right-table columns with NULL — making it the tool for "find rows with no match" queries. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **LEFT JOIN** — returns all rows from the left table (table1), and only the matched rows from the right table (table2); unmatched right-side columns become NULL. [S1]
+- **LEFT JOIN == LEFT OUTER JOIN** — the `OUTER` keyword is optional and does not change behavior. [S1]
+- **Syntax** — `SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;`. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **NULL-filter for anti-join** — adding `WHERE Orders.CustomerID IS NULL` after a LEFT JOIN isolates rows from the left table that have NO match in the right table (e.g. customers who never placed an order) — a common "anti-join" pattern. [S1]
+
+## 📖 세부 내용 (Details)
+- All customers with their orders, including customers with none: `SELECT Customers.CustomerName, Orders.OrderID FROM Customers LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID ORDER BY Customers.CustomerName;`. [S1]
+- Customers with NO orders (anti-join via NULL filter): `SELECT Customers.CustomerName, Orders.OrderID FROM Customers LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID WHERE Orders.CustomerID IS NULL;`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+소스에서 모순되는 정보는 발견되지 않음.
+
+## 🛠️ 적용 사례 (Applied in summary)
+현재 발견된 실제 적용 사례가 없습니다 — "주문 없는 고객 찾기" 패턴은 실무에서 흔한 리포팅 쿼리 형태다. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+All rows from left table (SQL):
+```sql
+SELECT Customers.CustomerName, Orders.OrderID
+FROM Customers
+LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID
+ORDER BY Customers.CustomerName;
+```
+Anti-join: left rows with no right match (SQL):
+```sql
+SELECT Customers.CustomerName, Orders.OrderID
+FROM Customers
+LEFT JOIN Orders
+ON Customers.CustomerID = Orders.CustomerID
+WHERE Orders.CustomerID IS NULL;
+```
+
+## ✅ 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 Joins]], [[SQL Inner Join]], [[SQL Right Join]], [[SQL Null Values]]
+- **참조 맥락:** 왼쪽 테이블의 모든 행을 보존해야 하는 리포팅/anti-join 쿼리의 기본 도구.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — SQL LEFT JOIN — https://www.w3schools.com/sql/sql_join_left.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "SQL LEFT JOIN" page (Astra wiki-curation, P-Reinforce v3.1 format).