---
id: sql-null-values
title: "SQL Null Values"
category: "Database"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["NULL values", "IS NULL", "IS NOT NULL", "SQL 널 값"]
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", "null"]
raw_sources: ["https://www.w3schools.com/sql/sql_null_values.asp"]
applied_in: []
github_commit: ""
---

# [[SQL Null Values]]

## 🎯 한 줄 통찰 (One-line insight)
NULL represents an unknown or absent value — distinct from zero or an empty string — and can only be tested with IS NULL / IS NOT NULL, never with `=`, `<`, or `<>`. [S1]

## 🧠 핵심 개념 (Core concepts)
- **NULL value** — represents unknown, missing, or inapplicable data; a placeholder for absence of data, not a value itself. [S1]
- **NULL ≠ 0 or ''** — a NULL field is one left blank at record creation, distinct from zero or an empty string. [S1]
- **Comparison operators don't work on NULL** — `=`, `<`, `<>` cannot test for NULL; use `IS NULL` / `IS NOT NULL` instead. [S1]
- **Syntax** — `SELECT column_names FROM table_name WHERE column_name IS NULL;` and the `IS NOT NULL` counterpart. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Dedicated NULL-test operators** — because ordinary comparison operators silently fail to match NULL, SQL provides `IS NULL`/`IS NOT NULL` as the only reliable test — a common footgun for beginners who try `WHERE column = NULL`. [S1]

## 📖 세부 내용 (Details)
- Find rows with a NULL field: `SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL;`. [S1]
- Find rows with a non-NULL field: `SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NOT NULL;`. [S1]

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

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — LEFT JOIN의 anti-join 패턴(`WHERE Orders.CustomerID IS NULL`)에서 이미 이 문법이 선행 사용되었다. [S1]

## 💻 코드 패턴 (Code patterns)
Test for NULL / NOT NULL (SQL):
```sql
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address 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 Where]], [[SQL Left Join]], [[SQL Null Functions]]
- **참조 맥락:** NULL을 다뤄야 하는 모든 쿼리의 기초 — 이후 SQL Null Functions(COALESCE 등)에서 NULL 대체 함수로 확장.

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

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