What is the average number of orders per day?
SELECT AVG( orders_num ) FROM ( SELECT created_at, COUNT( DISTINCT order_id ) orders_num FROM `sales_flat_order_item` GROUP BY CAST( created_at AS DATE ) )orders_per_day
What is the average number of orders per month?
SELECT AVG( orders_num ) FROM ( SELECT created_at, COUNT( DISTINCT order_id ) orders_num FROM `sales_flat_order_item` GROUP BY MONTH( created_at ) )orders_per_month