sum /avg an alias column

select sum(days_in_received_and_billed)/COUNT(days_in_received_and_billed) as avg_stuff


from
(

select
datediff(day, ( select top 1  t_order_detail_event.dte_created
 from t_order_detail_event
 where t_order_detail_event.business_work_flow_event_ID = 59 and t_order_detail_event.order_detail_ID = 1756117) ,
  (select top 1  t_order_detail_event.dte_created
 from t_order_detail_event
 where t_order_detail_event.business_work_flow_event_ID = 101 and t_order_detail_event.order_detail_ID = 1756117) )as days_in_received_and_billed,
) as data_s

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