Sql Not Returning Any Data

Hello,

Im currently working on a asp.net file hosting wesite, and im being faced with some problems

I currently have 4 sql tables. DownloadTable, MusicTable, ImageTable and VideoTable. Each of those tables contain a UserName column, a fileSize column and sme other columns. What i want to do is add up all the values in the fileSize column, for each table, and then add them up with the tables, and return one value, and all this happens where the UserName column corresponds to the UserName of the currently logged on User.

I already have an sql statement that performs this exact thing. It is as follow

select TotalDownLoadSize = sum(DownloadSize) + (select sum(VideoSize) from VideoTable where ([UserName] = @UserName ) ) + (select sum(ImageSize) from Images where ([UserName] = @UserName) ) + (select sum(MusicSize) from MusicTable where ([UserName] = @UserName) ) from DownloadTable where ([UserName] = @UserName)

But the problem is that all of the tables have to have a value in there size columns for the corresponding user, for this sql statement to return something.

For example, lets say i logged in as jon. If, for the UserName jon, the sum of DownloadTable returned 200, the sum of VideoTable returned 300, the sum of MusicTable returned 100. The sql statement i stated above will return 4 instead of 600, if the sum of ImageTable returned zero.

Is there  way around this?

Im not sure if ive been cleared enough, please feel free to request more info as needed.

Thank you very much, and thx in advance.