---
id: sql-joins
title: "SQL Joins"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["SQL JOIN Clause", "SQL 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"]
raw_sources: ["https://www.w3schools.com/sql/sql_join.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Joins]]

## 🎯 한 줄 통찰 (One-line insight)
JOIN combines rows from two or more tables based on a related column between them, with four variants (INNER/LEFT/RIGHT/FULL) differing in which unmatched rows they keep. [S1]

## 🧠 핵심 개념 (Core concepts)
- **JOIN clause** — combines rows from two or more tables based on a related column. [S1]
- **(INNER) JOIN** — returns only rows with matching values in both tables. [S1]
- **LEFT (OUTER) JOIN** — returns all rows from the left table, plus matched rows from the right table. [S1]
- **RIGHT (OUTER) JOIN** — returns all rows from the right table, plus matched rows from the left table. [S1]
- **FULL (OUTER) JOIN** — returns all rows when there is a match in either table. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Related-column identification** — before writing a JOIN, identify the shared column between two tables (e.g. `CustomerID` appears in both "Orders" and "Customers"). [S1]

## 📖 세부 내용 (Details)
- Example relationship: the "Orders" table's `CustomerID` refers to the "Customers" table's `CustomerID`. [S1]
- INNER JOIN combining the two: `SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;`. [S1]
- The four JOIN types are summarized as covering: matches-only (INNER), left-preserving (LEFT), right-preserving (RIGHT), and both-preserving (FULL). [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
소스에서 모순되는 정보는 발견되지 않음.

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 이 문서는 개론이며, 각 JOIN 유형은 이어지는 개별 챕터(Inner/Left/Right/Full/Self Join)에서 상세히 다뤄진다. [S1]

## 💻 코드 패턴 (Code patterns)
Basic INNER JOIN (SQL):
```sql
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;
```

## ✅ 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 Inner Join]], [[SQL Left Join]], [[SQL Right Join]], [[SQL Full Join]], [[SQL Aliases]]
- **참조 맥락:** JOIN 계열 챕터 전체의 개론 — 이후 4가지 JOIN 유형이 각각 별도 문서로 심화된다.

## 📚 출처 (Sources)
- [S1] W3Schools — SQL Joins — https://www.w3schools.com/sql/sql_join.asp

## 📝 변경 이력 (Change history)
- 2026-07-04: Initial draft synthesized from the W3Schools "SQL Joins" page (Astra wiki-curation, P-Reinforce v3.1 format).