Don't aggregate the values on your Widget table, just group by them.
SELECT W.ID,
W.Group,
SUM( WD.Payment) AS Payment,
W.Invoice
FROM Widget W
JOIN WidgetDetail WD ON W.ID = WD.ID
GROUP BY W.ID, W.Group, W.Invoice;
Luis C.
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How to post data/code on a forum to get the best help: Option 1 / Option 2