SELECT
costing_calc.costing_calcid,
clients.company_name,
costing_calc.requestdate,
costing_calc.requesttime,
CONCAT(costing_calc.order_number,'',LPAD(sales_order.sales_orderid,8,'0')) as 'order_number',
costing_calc.customer_order_no,
costing_calc.go_ahead,
order_type.description,
sales_order.order_complete_closed
FROM
costing_calc 
Left Join order_type ON order_type.order_typeid = costing_calc.order_typeid
left Join clients ON  clients.clientsid = costing_calc.clientsid
Left Join sales_order ON sales_order.costing_calcid = costing_calc.costing_calcid