Debug DISTINCT Masking Join Explosion

Fix query and remove anti-pattern: SELECT DISTINCT c.customer_id FROM customers c JOIN orders o ON o.customer_id=c.customer_id JOIN order_items i ON i.order_id=o.order_id WHERE i.sku='SKU-9';

customers(customer_id), orders(order_id, customer_id), order_items(order_id, sku)