Customer Order Summary — Solution & ExplanationProblems / Customer Order Summary / EditorialApproach

LEFT JOIN customers to orders on customer_id. This ensures customers with no orders still appear.

Aggregate: COUNT(order_id) correctly returns 0 for unmatched rows (since order_id is NULL). SUM and AVG on NULLs produce NULL, so wrap with COALESCE(..., 0).

Order by total_amount descending.

Key InsightUsing COUNT(order_id) instead of COUNT(*) is important — COUNT(*) would return 1 for customers with no orders because the LEFT JOIN still produces one row.Hints

Use LEFT JOIN to include customers without orders.

Use COUNT(o.order_id) instead of COUNT(*) so NULLs from the left join produce 0.

Use COALESCE to handle NULL sums and averages for customers with no orders.

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