Context

At companies like FleetOps, operational records often come from multiple systems such as warehouse apps, delivery tools, and support platforms. Analysts need a consistent way to combine these datasets before reporting or downstream processing.

Core question

Explain how you would use UNION and UNION ALL to combine operational data from multiple sources. In your answer, cover:

1. The difference between UNION and UNION ALL
2. How to align columns, data types, and business meaning across sources
3. When deduplication is appropriate and when it is risky
4. Common issues such as mismatched schemas, duplicate records, and inconsistent filters

Scope guidance

The interviewer expects a practical explanation, not just syntax. You should describe how to standardize source datasets, choose the right set operator, and prepare the combined result for reporting or aggregation in PostgreSQL.