[Shur]

Экономно так :

select sum(AmountCurCredit)
        from LedgerJournalTrans
        join [COLOR="Red"]TableID from[/COLOR]
            SalesPaymentJournal
        where SalesPaymentJournal.JournalNum == LedgerJournalTrans.JournalNum
           && SalesPaymentJournal.SalesId == this.SalesId;