Context

Leadership reports often drive decisions, so a small SQL mistake can create a large business problem. Interviewers ask this to see whether you have a disciplined process for validating query outputs, not just writing queries.

Question

Explain the steps you take to verify the accuracy of a report before presenting it to leadership. In your answer, cover how you validate source data, confirm business logic, test filters and aggregations, and check for edge cases such as nulls, duplicates, or missing records.

Scope Guidance

Keep the answer practical and SQL-focused. The interviewer expects a structured validation approach: how you compare results to known baselines, how you sanity-check totals and row counts, and how you make sure the final output matches the metric definition and reporting period.

Metric Definition Validation

Before trusting a report, confirm the exact business definition of each metric. Many reporting errors come from misinterpreting whether a metric should count rows, distinct entities, net values, or only records within a specific status or date range.

SELECT COUNT(DISTINCT customer_id) AS active_customers
FROM orders
WHERE order_date >= DATE '2024-01-01'
  AND order_date < DATE '2024-02-01';

Row Count and Aggregate Reconciliation

A reliable validation step is to reconcile report totals against simpler reference queries. If a grouped report rolls up to a total that does not match a direct aggregate from the base table, the logic likely has a filtering, grouping, or duplication issue.

SELECT SUM(amount) AS total_revenue
FROM orders
WHERE status = 'completed';

Edge Case Testing

Reports should be tested against null values, duplicate records, zero amounts, and boundary dates. These cases often expose hidden assumptions in WHERE clauses, GROUP BY logic, or date filtering.

SELECT COUNT(*) AS null_region_rows
FROM sales_report_source
WHERE region IS NULL;

Sample-Based Spot Checks

In addition to aggregate checks, inspect a small set of underlying records and manually trace whether they appear correctly in the report. This helps catch logic errors that totals alone may not reveal.

SELECT order_id, customer_id, amount, order_date
FROM orders
WHERE order_id IN (101, 205, 319);

Output Presentation Review

Even if the SQL logic is correct, the final report can still mislead if labels, sort order, date ranges, or rounding are unclear. Validation includes making sure the result is interpretable and aligned with what leadership expects to see.

SELECT region, ROUND(SUM(amount), 2) AS revenue
FROM orders
GROUP BY region
ORDER BY revenue DESC;