Write a SQL query to find the latest successful transaction per Stella.

To retrieve the most recent successful transaction for each Stella, you can use a window function or a correlated subquery. For example, with a window function: SELECT FROM (SELECT , ROW_NUMBER() OVER (PARTITION BY stella_id ORDER BY transaction_date DESC) AS rn FROM transactions WHERE status = 'successful') t WHERE rn = 1; This partitions the rows by stella_id, orders them by transaction_date descending, and assigns a row number. The outer query filters to the first row per partition, giving the latest successful transaction. Alternatively, a correlated subquery: SELECT t.* FROM transactions t WHERE t.status = 'successful' AND t.transaction_date = (SELECT MAX(transaction_date) FROM transactions WHERE stella_id = t.stella_id AND status = 'successful'); Both approaches run in O(n) time with proper indexes on stella_id and transaction_date. Indexing these columns ensures the database can quickly locate the relevant rows, and the window function approach is often more readable and efficient on modern RDBMS engines.