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select id,greatest(date1,date2,date3....)  closed_date
from YOUR_TABLE                                                                                                                                                                                    

Try this..

select id,greatest(nvl(date1,sysdate-36500),
                         nvl(date2,sysdate-36500),
                         nvl(date3,sysdate-36500)....)  closed_date
from YOUR_TABLE                                                                                                                                                                                                                                                                                                                                                                                                                    

so use the NVL on each column. and set it to
something ridiculous, so it will always be less than
the other values. and then, if the ridiculous value
shows up, it means all the columns were null for that
row.

Rather than relying on some rediculously small date use coalecse on each date column with the date column of intereste listed first. The order of the remaining terms in the coalesce statement is unimportant. e.g. on a table with three date columsn:

with x as (
  select 1 id, sysdate dte1, sysdate-1 dte2, sysdate+1 dte3 from dual union
  select 2, sysdate, null, null from dual union
  select 3, null, sysdate-1, null from dual union
  select 4, null, sysdate-1, sysdate+1 from dual union
  select 5, null, null, null from dual
)
select id, 
       greatest( coalesce(dte1,dte2,dte3), 
                 coalesce(dte2,dte3,dte1), 
                 coalesce(dte3,dte1,dte2)) Max_Date
from x
 
ID                     MAX_DATE                  
---------------------- ------------------------- 
1                      09-OCT-2007 12.15.44      
2                      08-OCT-2007 12.15.44      
3                      07-OCT-2007 12.15.44      
4                      09-OCT-2007 12.15.44      
5                      (null)
 
5 rows selecte

This way if they are all null, you get a null result, and don't need to handle your null value date as a special case. It also avoids the situation where the unlikely event occurs where your special null value date appears as actual data.