Context

At companies like Stripe or Airbnb, join choice often determines whether missing or unmatched records are visible during analysis. This becomes especially important when troubleshooting row-count drops, nulls, or unexpected gaps in reports.

Core question

Explain the difference between an INNER JOIN and a LEFT JOIN in SQL. In your answer, cover:

1. What rows each join returns
2. How unmatched rows are handled
3. When you would use each join in normal analysis
4. Why LEFT JOIN is often useful when debugging a data issue such as missing orders, failed mappings, or incomplete dimension data

Scope guidance

The interviewer expects a practical explanation, not just definitions. You should describe the result-set behavior, mention NULL handling, and give a simple troubleshooting example showing how the wrong join can hide the problem you are trying to investigate.