Using sub-resources, you get something like this:
GET /orders/1/clients
SELECT * FROM orders WHERE client_id = 1
And for
GET /clients/1/orders
it seems like in practice it would be the same relation since it would be wasteful to store two tables. Is this roughly correct? SELECT * FROM clients WHERE client_id IN (SELECT client_id FROM orders WHERE order_id = @order_id)
The GET /clients/1/orders is what would result in your query (for client 1 list all orders).