Обсуждение: SQL query question

I've been having problems with this query.  The second sub-query will not
always return data.
When that is the case, main query will not return a value, even though the
first query does.
Would anyone have any suggestions as to how I could return the
events.EventCost even if
registration.registrationRegCost is null?

SELECT (events.EventCost + registration.registrationRegCost) AS subtotal
FROM (SELECT [Event Registration].RegID AS eventRegID,
              SUM(Events.EventCost) AS EventCost
              FROM Events INNER JOIN
              [Event Registration] ON
              Events.ID = [Event Registration].EventID
              WHERE [Event Registration].RegID = 399
              GROUP BY [Event Registration].RegID) AS events,

            (SELECT dbo.[Registration Information].RegID AS
registrationRegID,
            dbo.[Registration Types].RegCosts AS registrationRegCost
            FROM dbo.[Registration Information],
            dbo.[Registration Types]
             WHERE dbo.[Registration Information].RegType =
dbo.[Registration Types].RegTypeID
            AND dbo.[Registration Information].RegID = @regid2) AS
registration