I have two tables Customers and Customer_orders. I'm trying to SELECT customer's order by using Customer_ID from Customer table.
SELECT * FROM Customer_orders WHERE Customer_ID = SELECT ID FROM Customers WHERE Customer_name = 'John Doe'
You need to JOIN those 2 tables and then query for what you need. Like this:
SELECT co.* FROM Customer_orders co INNER JOIN Customers c ON co.Customer_ID = c.ID WHERE c.Customer_name = 'John Doe';