SQL Subqueries on the from clause

Subqueries on the From clause

use AdventureWorks
go


select avg(t.total)
 from (
  select sum(sh.TotalDue) total,sh.CustomerID
  from Sales.SalesOrderHeader sh
  where OrderDate between '20040101' and '20041231'
  group by CustomerID
  ) as t  

declare @range datetime
set @range = '20030101'
select AVG(a.total) averageCustSpend,
 (select COUNT(*)
  from Sales.SalesOrderHeader
  where OrderDate between @range and DATEADD(year,1,@range)
 )as count
from(
 select SUM(sh.TotalDue) total,sh.CustomerID
 from Sales.SalesOrderHeader sh
 where OrderDate between @range and DATEADD(year,1,@range)
 group by CustomerID
 )a