Context

Teams often store similar records in separate tables or queries, such as regional sales, archived transactions, or data from different business units. Interviewers ask about UNION to test whether you understand how to stack compatible result sets correctly.

Core Question

Explain how you would use UNION to combine datasets with similar structure. In your answer, cover:

1. What requirements the two queries must satisfy
2. The difference between UNION and UNION ALL
3. When duplicate removal is desirable or harmful
4. How you would align column names, data types, and filters before combining results

Scope Guidance

The interviewer is usually looking for a practical explanation with a small SQL example, not just a definition. You should be able to describe the syntax, common mistakes, and how this applies to reporting or data consolidation tasks.