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Explain Coalesce vs ISNULL. What are the differences in SQL?

SQLeasy0.4 min read

**Architectural Logic**: COALESCE is standard; ISNULL is SQL Server-specific—portability and flexibility differ. **COALESCE**: Standard SQL; returns first non-NULL from list; `COALESCE(a, b, c)`. **ISNULL**: SQL Server; two args only; `ISNULL(a, b)`. **Equivalence**: For two...

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Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Fractal
Key Concepts Tested
sql

Why This Question Matters

This easy-level SQL question appears frequently in data engineering interviews at companies like Fractal. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (sql) will help you answer variations of this question confidently.

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Start by clearly defining the core concept being asked about. Interviewers want to see that you understand the fundamentals before diving into implementation details. Structure your answer with a definition, then explain the practical application with a concise example.

Expert Answer
83 words

Architectural Logic: COALESCE is standard; ISNULL is SQL Server-specific—portability and flexibility differ. COALESCE: Standard SQL; returns first non-NULL from list; COALESCE(a, b, c). ISNULL: SQL Server; two args only; ISNULL(a, b). Equivalence: For two args, same result. Differences: COALESCE portable; handles multiple fallbacks. ISNULL: type of first arg; COALESCE can change type. Example: COALESCE(phone, email, 'N/A') vs ISNULL(phone, 'N/A'). Best Practice: COALESCE for portability and chains; ISNULL when SQL Server and two args suffice. Edge Case: Type handling differs (e.g., VARCHAR vs INT).

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