In SQL, suppose you have a table with customer ID, transaction date, and amount. How would you find the latest transaction for each customer? Explain your approach.

To get the latest transaction per customer you can use a window function or a correlated subquery. Example with window function: SELECT customer_id, transaction_date, amount FROM ( SELECT t., ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY transaction_date DESC) AS rn FROM transactions t ) sub WHERE rn = 1; This assigns a row number per customer ordered by date descending and keeps only the first row. Complexity is O(n) time and O(n) space for the window. Alternatively, a correlated subquery: SELECT t. FROM transactions t WHERE t.transaction_date = (SELECT MAX(transaction_date) FROM transactions WHERE customer_id = t.customer_id); This runs in O(n log n) if indexes exist. Both approaches return the most recent transaction for each customer.