SQL newbie

I have a query which returns the correct resultset:

SELECT invoicedetail.itemtype_id, itemtype.itemname,
AVG(CASE WHEN invoice.isinvoice THEN invoicedetail.totalitemcost END) AS
averagecost,
COUNT(CASE WHEN invoice.isinvoice THEN itemtype.itemname END) AS numsales,
COUNT(CASE WHEN NOT invoice.isinvoice THEN itemtype.itemname END) AS
numcredits,
SUM(CASE WHEN invoice.isinvoice THEN invoicedetail.totalitemcost ELSE 0.00
END) AS totalcost,
SUM(CASE WHEN NOT invoice.isinvoice THEN invoicedetail.totalitemcost ELSE
0.00 END) AS totalcredit

FROM itemtype, invoicedetail, invoice

WHERE itemtype.itemtype_id=invoicedetail.itemtype_id
AND invoicedetail.invoice_id=invoice.invoice_id
AND itemtype.itemtype_id=1

GROUP BY invoicedetail.itemtype_id, itemtype.itemname

How every when I change it to this:

SELECT invoicedetail.itemtype_id, itemtype.itemname,
AVG(CASE WHEN invoice.isinvoice THEN invoicedetail.totalitemcost END) AS
averagecost,
COUNT(CASE WHEN invoice.isinvoice THEN itemtype.itemname END) AS numsales,
COUNT(CASE WHEN NOT invoice.isinvoice THEN itemtype.itemname END) AS
numcredits,
SUM(CASE WHEN invoice.isinvoice THEN invoicedetail.totalitemcost ELSE 0.00
END) AS totalcost,
SUM(CASE WHEN NOT invoice.isinvoice THEN invoicedetail.totalitemcost ELSE
0.00 END) AS totalcredit,
COUNT(inventory.itemtype_id) AS totalstock **** CHANGED LINE

FROM itemtype, invoicedetail, invoice, inventory **** CHANGED LINE

WHERE itemtype.itemtype_id=invoicedetail.itemtype_id
AND invoicedetail.invoice_id=invoice.invoice_id
AND invoicedetail.itemtype_id=inventory.itemtype_id **** CHANGED LINE
AND itemtype.itemtype_id=1

GROUP BY invoicedetail.itemtype_id, itemtype.itemname

The original columns are multiplied by 8000, coincedently the number of
items in inventory that have itemtype_id=1.

Basically what I want is the original columns plus the number of items in
inventory

Can anyone help

TIA

David McCallum