sum function

Select 
   Q.CompanyName,
   Q.CompAddress,
   Q.ClientName,
   Q.Total,
   QD.TotQuantity
 from Quotation Q  inner join
      (select QuoteNumber, sum(Quantity) as TotQuantity
       from QuoteDetail where QuoteNumber=4
       group by QuoteNumber) as QD
            on (Q.QuoteNo=QD.QuoteNumber)
 where Q.QuoteNo=4

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Belo Horizonte, MG - Brasil