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SQL Aggregation Interview Questions
Aggregation questions test whether you can summarize data at the right grain using GROUP BY and HAVING.
Common themes include COUNT vs COUNT(DISTINCT), conditional aggregation with CASE, and aggregating after joins.
Interviewers look for correct groupings, handling of NULLs, and clear explanation of your logic.
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Common SQL aggregation interview patterns
- Choosing the correct grouping level
- HAVING vs WHERE for post-aggregation filters
- Conditional aggregation with CASE
- COUNT(DISTINCT) vs COUNT
- Aggregation after joins
- Top N per group with aggregation
SQL aggregation interview questions
Filter and sort
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Common mistakes with SQL aggregation
- Grouping by too many columns
- Using WHERE instead of HAVING for aggregates
- Double counting after joins
- COUNT(column) skipping NULLs unintentionally
- Misusing DISTINCT on aggregated fields
How aggregation questions are evaluated in interviews
Correct grouping and filtering matter more than micro-optimizations.
Explain your grain, especially when combining joins and aggregates.
Call out assumptions about NULLs and duplicates.