Join choice directly affects row counts, missing values, and business conclusions in analytics work. In card member and merchant reporting, using the wrong join can silently exclude important records.
Explain the difference between an INNER JOIN and a LEFT JOIN in PostgreSQL. You should describe what rows each join returns, how unmatched rows are handled, and how NULL values appear in the result. Use a simple example such as joining Amex card members to transactions or offers to redemptions.
The interviewer is looking for a clear conceptual explanation rather than a complex query. A strong answer should define both joins, compare their outputs, explain when you would choose each one, and mention a common mistake such as filtering a LEFT JOINed table in the WHERE clause and accidentally turning it into an INNER JOIN.
