How do you check that the numbers from your SQL query are correct before you share them?

What the interviewer wants to hear

• Checks row counts and uniqueness of keys before and after joins
• Compares totals with a trusted source or a previous report
• Spot-checks a few individual records by hand
• Looks for NULLs, duplicates and odd values
• Asks whether the result makes business sense

Model answer

I check in layers. First, row counts: I count rows in each table and after each join, and confirm the join keys are unique where I expect them to be, because a duplicated key silently inflates totals. Second, I reconcile a headline number such as last month revenue with a trusted source like the finance report or an existing dashboard. Third, I pick three or four real customers or orders and trace them by hand through the query to see that each step does what I think. I also scan for NULLs, negative amounts, future dates and test accounts. Finally, I ask if the result makes sense: if conversion suddenly doubled, I assume a bug before a miracle. When something still looks uncertain, I say so clearly in the summary instead of hiding it.

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