min(column) with nulls?
MIN will return the smallest non-NULL value.  The query won't throw an error, it will simply ignore the NULL values

SQL&gt; with x as (
  2    select null dt from dual union all
  3    select sysdate from dual
  4  )
  5  select min(dt)
  6    from x;

MIN(DT)
---------
21-OCT-11