First Post: Problem With SQL Query

I'm putting together a .NET web page for my group at work to use for server and infrastructure equipment inventory. I'm having a great deal of trouble with one query. The query needs to pull from three tables; 'Rack', 'Server', and 'Infrastructure.' What I need it to do is SUM the number of rack units (slots) used by servers and infrastructure equipment, grouped by the rack number, and subtract it from the number of units the rack has available. I have a query that works for a single rack...

SELECT [Server].[Rack_No], ([Available_Slots] - (SUM([Rack_Units_Used]) + (SELECT SUM([Rack_Units_Used]) AS [RUs]
FROM [Infrastructure] WHERE [Infrastructure].[Rack_No] = 'Rack1'))) AS [RUs]
FROM [Server], [Rack]
WHERE [Server].[Rack_No] = 'Rack1' AND [Rack].[Rack_No] = 'Rack1'
GROUP BY [Server].[Rack_No], [Available_Slots];

...but, I cannot seem to put one together that will generate a report of all the racks. The closest I've come is this query...

SELECT [Rack].[Rack_No], ([Available_Slots] - (SUM([Infrastructure].[Rack_Units_Used]) + (SUM([Server].[Rack_Units_Used])))) AS [RUs]
FROM [Server], [Rack], [Infrastructure]
WHERE [Server].[Rack_No] = [Rack].[Rack_No] AND [Infrastructure].[Rack_No] = [Rack].[Rack_No]
GROUP BY [Rack].[Rack_No], [Available_Slots]
ORDER BY [Rack].[Rack_No];

But I have to problems. First, the results are completely inaccurate. Second, It cannot deal with null values if there isn't at least one server AND at least one piece of infrastructure equipment installed in a given rack. I need it to return a '0' instead of 'NULL' if that is the case.