SQL Data Validation During Testing

Context

Data teams at companies like Stripe use SQL to verify that loaded or transformed data matches expectations before dashboards, reports, or downstream models rely on it. Interviewers ask this to assess whether you can use SQL as a practical testing tool, not just for analysis.

Question

Explain how you would use SQL to verify data during testing. In your answer, cover:

1. What kinds of checks you would run on a table after a load or transformation
2. How SQL can validate completeness, uniqueness, null handling, and business rules
3. Examples of simple validation queries you would write
4. How you would investigate mismatches when expected and actual results differ

Scope Guidance

Keep the answer focused on common SQL-based validation patterns used in analytics engineering, ETL testing, or QA. The interviewer is looking for a structured approach, clear examples, and awareness of common failure modes rather than advanced database internals.