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title

SQL Select Distinct

🎯 한 줄 통찰 (One-line insight)

SELECT DISTINCT returns only unique values from a column, collapsing duplicate rows that a plain SELECT would otherwise repeat. [S1]

🧠 핵심 개념 (Core concepts)

SELECT DISTINCT — returns only distinct (unique) values, removing duplicate rows from the result. [S1]
Syntax — SELECT DISTINCT column1, column2, ... FROM table_name;. [S1]
COUNT(DISTINCT column) — counts the number of unique values in a column, but is not supported in Microsoft Access. [S1]

🧩 추출된 패턴 (Extracted patterns)

Dedup-then-count workaround — for databases lacking COUNT(DISTINCT ...) (e.g. MS Access), wrap a SELECT DISTINCT subquery and COUNT(*) the outer result: SELECT Count(*) AS DistinctCountries FROM (SELECT DISTINCT Country FROM Customers);. [S1]

📖 세부 내용 (Details)

Without DISTINCT, a query returns the column's value from every record, including repeats: SELECT Country FROM Customers;. [S1]
With DISTINCT, only unique values are returned: SELECT DISTINCT Country FROM Customers;. [S1]
Counting unique values directly: SELECT COUNT(DISTINCT Country) FROM Customers; — not supported on MS Access, which needs the subquery workaround above. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

DBMS 차이: COUNT(DISTINCT column_name)이 MS Access에서는 지원되지 않는다는 점이 명시적으로 언급됨 — 방언별 차이가 존재. [S1]

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — 이후 COUNT() 챕터에서 DISTINCT와의 조합이 더 다뤄진다. [S1]

💻 코드 패턴 (Code patterns)

Select unique values (SQL):

SELECT DISTINCT Country FROM Customers;

Count unique values, MS Access-safe (SQL):

SELECT Count(*) AS DistinctCountries
FROM (SELECT DISTINCT Country FROM Customers);

✅ 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 Select, SQL Count, SQL Group By
참조 맥락: SELECT 결과의 중복을 제거해야 할 때 참조하는 문서 — COUNT()와 결합해 고유값 개수를 셀 때 자주 함께 쓰인다.

📚 출처 (Sources)

[S1] W3Schools — SQL SELECT DISTINCT Statement — https://www.w3schools.com/sql/sql_distinct.asp

📝 변경 이력 (Change history)

2026-07-04: Initial draft synthesized from the W3Schools "SQL SELECT DISTINCT Statement" page (Astra wiki-curation, P-Reinforce v3.1 format).

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