Write SQL queries to fetch user transaction data

A practical SQL exercise where you write queries against a users/transactions schema. Expect to demonstrate joins, filtering, aggregation, and ordering — e.g. fetch total transaction amount per user, find users with no transactions, or retrieve the most recent transaction per user.

Common concepts tested:

• JOIN types (INNER, LEFT) and when to use each
• Aggregation with GROUP BY / HAVING
• Window functions (ROW_NUMBER, RANK) for top-N per group
• Indexing considerations and query performance

Discussion may extend to database fundamentals: normalization, indexing strategy, or transaction isolation levels.