1. QUERY SQL SELECT c.id, c.name, c.country, SUM(o.amount) AS total_spent FROM customers AS c JOIN orders AS o ON c.id = o.customer_id WHERE o.status = 'completed' AND o.created_at >= DATE '2025-01-01' AND o.created_at < DATE '2026-01-01' GROUP BY c.id, c.name, c.country ORDER BY total_spent DESC LIMIT 10; 2. EXPLANATION Joins customers to orders using customers.id = orders.customer_id. Filters to: only orders with status = 'completed' only orders created during the 2025 calendar year. Groups the results by customer so all qualifying orders for each customer are aggregated. Uses SUM(o.amount) to calculate each customer's total spend. Sorts by total spend in descending order. Returns only the top 10 customers. Using a date range (>= '2025-01-01' and < '2026-01-01') is generally more efficient than extracting the year because it allows PostgreSQL to use an index on created_at. 3. ASSUMPTIONS orders.customer_id is a foreign key referencing customers.id. orders.amount contains the monetary value of each order. A completed order is represented by status = 'completed'. created_at is a DATE, TIMESTAMP, or TIMESTAMPTZ column. Only customers with at least one completed order in 2025 should appear in the results. 4. PERFORMANCE TIPS For a large orders table, consider these indexes: SQL CREATE INDEX idx_orders_status_created_customer ON orders (status, created_at, customer_id); Or, if completed orders are the overwhelming majority of queries: SQL CREATE INDEX idx_orders_completed_2025 ON orders (created_at, customer_id) WHERE status = 'completed'; Additional tips: Keep the date filter as a range instead of using EXTRACT(YEAR FROM created_at), since range predicates are index-friendly. Ensure customers.id is the primary key (typically indexed automatically). 5. VARIATION If you only need the customer ID and name (not country), you can simplify the grouping: SQL SELECT c.id, c.name, SUM(o.amount) AS total_spent FROM orders AS o JOIN customers AS c ON c.id = o.customer_id WHERE o.status = 'completed' AND o.created_at >= DATE '2025-01-01' AND o.created_at < DATE '2026-01-01' GROUP BY c.id, c.name ORDER BY total_spent DESC LIMIT 10; This version produces the same ranking while returning fewer customer attributes.