Context

Joins are one of the most common SQL topics in interviews because they directly affect which rows appear in a result set. A small change in join type can silently remove important records.

Core question

Explain the difference between INNER JOIN and LEFT JOIN using a real-world example such as customers and orders.

In your answer, cover:

1. What each join returns
2. How unmatched rows are handled
3. A business case where INNER JOIN is the correct choice
4. A business case where LEFT JOIN is the correct choice
5. What the result would look like when some customers have no orders

Scope guidance

The interviewer expects a practical explanation, not just definitions. Use simple PostgreSQL examples, describe the output in words, and mention common mistakes such as accidentally filtering out unmatched rows after a LEFT JOIN.