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What is the difference between SELECT, COUNT(*), and COUNT(1)?

SQLeasy0.5 min readPremium

SELECT * returns all columns. COUNT(*) counts all rows including NULLs. COUNT(1) counts rows using literal 1 (same as COUNT(*)). COUNT(column) counts non-NULL values in that column. In practice, COUNT(*) and COUNT(1) are equivalent—optimizers treat them alike. Use COUNT(*) for...

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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
EY

Why This Question Matters

This easy-level SQL question appears frequently in data engineering interviews at companies like EY. While less common, it tests deeper understanding that distinguishes strong candidates.

How to Approach This

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
95 words

SELECT returns all columns. COUNT() counts all rows including NULLs. COUNT(1) counts rows using literal 1 (same as COUNT()). COUNT(column) counts non-NULL values in that column. In practice, COUNT() and COUNT(1) are equivalent—optimizers treat them alike. Use COUNT() for total rows; COUNT(column) when excluding NULLs matters. Best practice: Prefer COUNT() for clarity; avoid COUNT(*) when you need non-NULL count of a specific column. Why it matters: Design choices compound at scale—wrong approach can cause 100× overhead. Scalability trade-offs: Profile before optimizing; validate on sample then full. Cost implications: Suboptimal choices multiply at billion-row scale.

The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations covering performance optimization and real-world examples.

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According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.

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