学步园

select c.sn, d.sn, c.due_date - d.due_date

from 

(select * FROM ImpDn where sn%2 = 0) c 

join (select * from ImpDn where sn%2 = 1) d 

on d.sn+1 = c.sn 

(select ROW_NUMBER() OVER (ORDER BY NEWID()) AS RowNo FROM ImpDn)