Join logic is fundamental in analytics work because most reporting depends on combining related datasets such as Card Members, transactions, merchants, and servicing records in American Express systems.
Explain the main types of SQL JOINs and when you would use each. You should cover how INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN, and SELF JOIN behave, what rows each returns, and how NULL values appear in the result. It is also helpful to mention how UNION differs from a JOIN, since candidates sometimes confuse the two.
Answer at an interview level: define each join clearly, describe a practical use case, and call out common mistakes such as filtering away unmatched rows after an outer join. You do not need to discuss optimizer internals, but you should be precise about result sets and business implications.