When you combine result sets in analytics or reporting workflows, choosing between UNION and UNION ALL affects both correctness and runtime. This comes up often when merging transaction extracts, campaign audiences, or Amex offer activity from multiple sources.
Explain the difference between UNION and UNION ALL in SQL. You should describe how each handles duplicate rows, what that means for the final result set, and why one is usually faster than the other. Also explain the kinds of scenarios where you would prioritize UNION for correctness versus UNION ALL for performance.
Keep your answer practical. The interviewer is looking for a clear explanation of result-set behavior, duplicate handling, and performance trade-offs, plus a few examples of when duplicate rows are expected, acceptable, or harmful in downstream analysis.
When you combine result sets in analytics or reporting workflows, choosing between UNION and UNION ALL affects both correctness and runtime. This comes up often when merging transaction extracts, campaign audiences, or Amex offer activity from multiple sources.
Explain the difference between UNION and UNION ALL in SQL. You should describe how each handles duplicate rows, what that means for the final result set, and why one is usually faster than the other. Also explain the kinds of scenarios where you would prioritize UNION for correctness versus UNION ALL for performance.
Keep your answer practical. The interviewer is looking for a clear explanation of result-set behavior, duplicate handling, and performance trade-offs, plus a few examples of when duplicate rows are expected, acceptable, or harmful in downstream analysis.
