Problem
Context
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.
Core question
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.
Scope guidance
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.
Practicing as: Data Analyst interview at American ExpressHi, I'll play your American Express interviewer for the Data Analyst role. Candidates describe these interviews as mostly positive and moderately difficult, so expect me to be friendly and conversational. Take your time with the question above and answer like we're in the room.
You are practicing as a guest. Sign up free to get your answer graded with AI feedback. Your draft stays right here.
