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76 lines
3.5 KiB
Markdown
76 lines
3.5 KiB
Markdown
---
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id: sql-count
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title: "SQL Count"
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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 COUNT() Function", "COUNT function", "SQL 개수 세기"]
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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", "count", "aggregate"]
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raw_sources: ["https://www.w3schools.com/sql/sql_count.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[SQL Count]]
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## 🎯 한 줄 통찰 (One-line insight)
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COUNT() behaves differently depending on its argument — `COUNT(*)` counts all rows including NULLs, `COUNT(column)` counts non-null values, and `COUNT(DISTINCT column)` counts unique non-null values. [S1]
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## 🧠 핵심 개념 (Core concepts)
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- **COUNT() function** — returns the number of rows matching a specified criterion. [S1]
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- **Syntax** — `SELECT COUNT([DISTINCT] column_name | *) FROM table_name WHERE condition;`. [S1]
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- **`COUNT(*)`** — counts the total number of rows, including NULL values. [S1]
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- **`COUNT(column_name)`** — counts only non-null values in that column. [S1]
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- **`COUNT(DISTINCT column_name)`** — counts only unique, non-null values. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Argument choice changes semantics** — the three COUNT variants answer three different questions (total rows / filled-in rows / unique values), so picking the right one matters for correctness, not just style. [S1]
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## 📖 세부 내용 (Details)
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- Total row count: `SELECT COUNT(*) FROM Products;`. [S1]
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- Non-null count for a column: `SELECT COUNT(ProductName) FROM Products;`. [S1]
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- Unique non-null count: `SELECT COUNT(DISTINCT Price) FROM Products;`. [S1]
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- With WHERE: `SELECT COUNT(ProductID) FROM Products WHERE Price > 20;`. [S1]
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- With alias: `SELECT COUNT(*) AS [Number of records] FROM Products;`. [S1]
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- With GROUP BY: `SELECT COUNT(*) AS [Number of records], CategoryID FROM Products GROUP BY CategoryID;`. [S1]
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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소스에서 모순되는 정보는 발견되지 않음.
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## 🛠️ 적용 사례 (Applied in summary)
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현재 발견된 실제 적용 사례가 없습니다 — SELECT DISTINCT 챕터에서 이미 소개된 `COUNT(DISTINCT ...)` 패턴이 여기서 정식으로 다뤄진다. [S1]
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## 💻 코드 패턴 (Code patterns)
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Three COUNT variants (SQL):
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```sql
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SELECT COUNT(*) FROM Products; -- all rows, incl. NULLs
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SELECT COUNT(ProductName) FROM Products; -- non-null values only
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SELECT COUNT(DISTINCT Price) FROM Products; -- unique non-null values
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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 Aggregate Functions]], [[SQL Select Distinct]], [[SQL Group By]]
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- **참조 맥락:** 행 개수를 세는 모든 리포트의 기본 함수 — DISTINCT와 결합해 고유값 개수도 셀 수 있다.
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## 📚 출처 (Sources)
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- [S1] W3Schools — SQL COUNT() Function — https://www.w3schools.com/sql/sql_count.asp
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## 📝 변경 이력 (Change history)
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- 2026-07-04: Initial draft synthesized from the W3Schools "SQL COUNT() Function" page (Astra wiki-curation, P-Reinforce v3.1 format).
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