【DB2】NULLS LAST与NULLS FIRST

DB2函数配合 
select row_number() over(order by col desc nulls first/nulls last ) as rn,col from table1order by rn; 
nulls first：把空值放到最前面，再对非空值排序 
nulls last：把空值放到最后面在对非空值排序