Getting a total of a number of group by results

[ZiggyShort]

I need to summate across a number of line items over a group of invoices, then round down on an invoice by invoice basis to the nearest penny.
I would then like to get a total of THOSE rounded values.
Does my sql need to return the items, and i then manually summate these values by reading each one by one, or can I do this in the sql. Remember, I still need to round on an invoice by invoice basis.
My sql which returns rounded invoice totals is given below:


SELECT
SUM(FLOOR(sii.unitPrice*sii.Qty)) AS total
FROM salesinvoiceItem sii,
salesinvoice si
WHERE sii.invoiceId = si.invoiceId
GROUP BY si.InvoiceId

;

This returns a number of rows which I then need a total for: is there an elegant way of achieving this in sql server in a query?

[ZiggyShort]

Sorted it:

SELECT SUM(total)
FROM
(SELECT
SUM(FLOOR(sii.unitPrice*sii.Qty)) AS total
FROM salesinvoiceItem sii,
salesinvoice si
WHERE sii.invoiceId = si.invoiceId
GROUP BY si.InvoiceId )somename