Troubleshooting Missing Rows with Joins | Dataford Interview Questions

Your question is Troubleshooting Missing Rows with Joins. Take a moment with it on the right.

Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).

Problem

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:

What rows each join returns
How unmatched rows are handled
When you would use each join in normal analysis
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.

Your question is Troubleshooting Missing Rows with Joins. Take a moment with it on the right.

Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).

Context

Core question

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

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