var customerDTOs = from customer in GetAllCustomers()
                   where customer.Email == "some@email.com"
                   select new CustomerDTO
                   {
                      Id = customer.Id,
                      Name = customer.Name,
                      OrderId = customer.Orders.Select(x => x.Id).FirstOrDefault()
                   }

select customers.Id, customers.Name,
(select orders.Id from order_customer, orders
 where customers.Id = order_customer.customerId
 and order_customer.orderId = orders.Id
 limit 1))
from customers
where customers.Email='some@email.com'

select customers.Id, customers.Name,
(select order_customer.orderId from order_customer
 where customers.Id = order_customer.customerId
 limit 1))
from customers
where customers.Email='some@email.com'