Hope I got the logic right:
create function x (@from datetime, @to datetime)
returns table
as
return (
with cust as (
select o.customerid, o.employeeid, sum(unitprice*quantity) as cust_purchase
from orders o
inner join [order details] d on o.orderid=d.orderid
where o.orderdate between @from and @to
group by o.customerid, o.employeeid
),
emp as (
select o.employeeid, avg(unitprice*quantity) as emp_sale
from orders o
inner join [order details] d on o.orderid=d.orderid
where o.orderdate between @from and @to
group by o.employeeid
)
select c.customerid, c.companyname, c.city, c.address
from cust
inner join emp on cust.employeeid = emp.employeeid
and cust.cust_purchase > emp.emp_sale
inner join customers c on cust.customerid = c.customerid
)
go
select * from x ('19980401', '19980430')