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where
	Invoice_Date between '1 Jan 1999' and '1 Jan 2000'
group by Customer.Cust_ID,Customer_Name

If the database does not support outer joins you have to write something
like:

select
	Customer.Cust_ID,
	Customer_Name,
	count(Invoice.Invoice_No) as InvCount,
	sum(Invoice.Total) as InvSum