2nd/10_Wiki/Topics/Domain_Programming/Topic_SQL/SQL_Exists.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-exists	SQL Exists	Database	draft	conceptual		SQL EXISTS Operator EXISTS operator SQL 존재 확인		B	0.9	2026-07-04	2026-07-04			sql database w3schools exists subquery	https://www.w3schools.com/sql/sql_exists.asp

SQL Exists

🎯 한 줄 통찰 (One-line insight)

EXISTS tests whether a correlated subquery returns any row at all, evaluating to TRUE the moment one match is found — a boolean existence check rather than a value comparison. [S1]

🧠 핵심 개념 (Core concepts)

• EXISTS operator — used in a WHERE clause to check whether a subquery returns any rows; TRUE if at least one row, FALSE otherwise. [S1]
• Syntax — SELECT column_name(s) FROM table_name WHERE EXISTS (subquery);. [S1]
• Correlated subquery — the subquery typically references the outer query's table (e.g. Products.SupplierID = Suppliers.supplierID) to test existence per outer row. [S1]

📖 세부 내용 (Details)

• Suppliers with at least one product priced under 10: [S1]

SELECT SupplierName
FROM Suppliers
WHERE EXISTS (
  SELECT ProductName
  FROM Products
  WHERE Products.SupplierID = Suppliers.supplierID AND Price < 10
);

• Same pattern, different threshold (Price = 22), illustrating that EXISTS only cares about row presence, not the queried column's actual values. [S1]

⚖️ 모순 및 업데이트 (Contradictions & updates)

소스에서 모순되는 정보는 발견되지 않음.

🛠️ 적용 사례 (Applied in summary)

현재 발견된 실제 적용 사례가 없습니다 — IN (SELECT ...)과 유사한 목적(서브쿼리 기반 필터링)이지만, EXISTS는 값이 아닌 행의 존재 여부만 확인한다는 차이가 있다. [S1]

💻 코드 패턴 (Code patterns)

Correlated EXISTS subquery (SQL):

SELECT SupplierName
FROM Suppliers
WHERE EXISTS (
  SELECT ProductName
  FROM Products
  WHERE Products.SupplierID = Suppliers.supplierID AND Price < 10
);

✅ 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 In, SQL Any, SQL All
• 참조 맥락: 서브쿼리 결과의 "존재 여부"만 확인하면 되는 필터링에 사용 — 값 비교가 필요하면 IN/ANY/ALL을 대신 고려.

📚 출처 (Sources)

• [S1] W3Schools — SQL EXISTS Operator — https://www.w3schools.com/sql/sql_exists.asp

📝 변경 이력 (Change history)

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