Back to the original topic, my preference for an OLTP system is to use NULL when there can be an unknown value.  For an OLAP type system such as a...

I'd start with a high level summary of all the things in tempDB before looking for specific sessions:
USE tempdb;
SELECT SUM(unallocated_extent_page_count) AS FreePages,
  CAST(SUM(unallocated_extent_page_count)/128.0 AS decimal(9,2))...

Does the ID column of Table1 have unique values?  If not, that could be a potential problem since you join Table1 to itself in the subquery.  If it is unique,...

You could try doing a CROSS APPLY such as in this article to operate on the data in a set based fassion:
http://www.sqlservercentral.com/articles/CROSS+APPLY+VALUES+UNPIVOT/91234/
Then you can do aggregates...

Again, I suppose the real issue is how much free space do you need on the disk, not how much unused space do you have in the database.  Have you...

To verify what files are used by a specific database and how much of them are used, you can run this script in the database:
SELECT DB_NAME() AS...

This may be a silly question, but is there a reason you can't schedule the SQL Agent job in SQL Agent?