[SQL] Difference in columns

Hi All,

I have a view that generates output similar to this.

select * from foo.view;

       ts          | size
-------------------+-----
 2002-03-16        | 11 
 2002-03-17        | 16 
  2002-03-18        | 18
 2002-03-19        | 12  

I am trying to find the difference between the size column. So the desired output would be

       ts          | size| Diff
-------------------+-----+------
 2002-03-16        | 11  | 0
  2002-03-17        | 15  | 4
 2002-03-18        | 18  | 3
 2002-03-19        | 12  | -6

I need the first column to be 0, since it will be 11-11. The second colum is 15-11. The third column is 18-15. The fourth column is 12-18.

Any thoughts about this?

TIA