Explain the difference between WHERE and HAVING clauses in SQL
This question tests the ability to filter data at different stages of query execution and aggregation.
Why Interviewers Ask This
Understanding the execution order of SQL clauses is crucial for writing efficient and correct queries. Candidates must distinguish between filtering raw rows and filtering aggregated groups.
How to Answer This Question
State that WHERE filters rows before grouping, while HAVING filters after aggregation. Explain that WHERE cannot use aggregate functions, whereas HAVING is designed for them. Provide a clear example contrasting the two.
Key Points to Cover
- WHERE: pre-aggregation filtering
- HAVING: post-aggregation filtering
- Aggregate function usage
- Execution order importance
Sample Answer
WHERE filters individual rows before any grouping or aggregation occurs, so it cannot use aggregate functions like SUM or COUNT. It is best for narrowing down raw data early, such as filtering by date range. In contrast,…
Common Mistakes to Avoid
- Using aggregates in WHERE
- Confusing the timing of execution
- Not providing examples
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