Rewrite a JOIN Query Without Using JOINs

Problem Given the schema below, answer the same question two ways — once without using any JOIN, and once using an explicit INNER JOIN — then explain how the two relate and where they differ.

Schema

• customers(customer_id, name, city)
• orders(order_id, customer_id, order_value) — related to customers on customer_id

Task

• Write both queries returning the same result set — e.g. every customer who has placed at least one order.
• Explain the logical equivalence and where it breaks: row fan-out, NULL handling, readability, and performance.