Join vs Union in Analytics

Context

Data analysts often need to either combine related datasets or append similar datasets together. Knowing whether to use a JOIN or a UNION is a basic but important SQL decision because the result shape and meaning are very different.

Core Question

Explain the difference between a JOIN and a UNION in SQL in the context of data analysis.

Your answer should address:

1. What each operation does to the result set
2. When an analyst should use a JOIN
3. When an analyst should use a UNION or UNION ALL
4. Common mistakes analysts make when choosing between them
5. A simple example of each using PostgreSQL-compatible SQL

Scope Guidance

The interviewer is looking for a practical explanation, not just syntax definitions. Focus on how these operations change the structure of data, what business questions they answer, and why using the wrong one can lead to incorrect analysis.