LEFT vs INNER JOIN Reporting

Context

In reporting at Meta, the join type you choose can change business conclusions. For example, a report on Facebook Pages, Instagram creators, or ad accounts may either include entities with no activity or exclude them entirely depending on the join.

Core question

Explain how LEFT JOIN and INNER JOIN differ in a reporting context. In your answer, discuss:

1. What rows each join returns
2. How unmatched records affect counts, sums, and conversion-style metrics
3. When a business analyst should prefer LEFT JOIN over INNER JOIN
4. A simple reporting example where using the wrong join would produce a misleading result

Scope guidance

The interviewer is looking for a practical analytics explanation, not just syntax definitions. Focus on how join choice changes report completeness, NULL handling, and interpretation of KPIs in recurring business reporting.