When doing a comparison that have null values, the best way is to convert it. Keeping the null values in aggration will always cause issues.

select a.id, a.desc1, b.desc1

from #tempa...

This is documented in BOL for null values;

"Null values cannot be used for information that is required to distinguish one row in a table from another row in a table"

Based...

When we specify the column names,  the sql will not count the null values even if the ansi_null off

select count(*) from #temp = 1

select count(a) from #tmp = 0