You need to convert the null values to zero, if that suits you. Like,
This converts the null values and eases up the calculations. You may also refer to Handling Null Values | Tableau Software
thanks. The problem is I cant use 0, and that is a valid field that I need. I need to somehow remove them completely.
If you want to remove nullable values from a set of columns in SQL you can use the COALESCE function, it's a multiple parameter ISNULL, so you can do COALESCE(Diff1,Diff2,Diff3,Diff4,...,DiffN) and it will return the first non-null. If you can't change the view you can pass it through using the RAWSQL_* functions (the provider would need to support COALESCE though). e.g. RAWSQL_STR("COALESCE(%1,%2...%n)",[Diff1],[Diff2],...[DiffN]) This assumes that you will only have one non-null value in a given row/set of columns, which I think fits your example.
You may be able to filter the nulls at the data source level using the "Edit Data Source Filters" option.