Handling Null in Greatest function in Oracle

Question

I want to compare two dates from two columns and get the greatest and then compare against a date value.The two column can hold NULL values too.For example I want the below OUTP

Answer 1

I tried this..found after googling

WITH ABC AS ( SELECT NULL AS col1 , NULL AS col2 FROM dual UNION
 SELECT NULL , DATE'2013-08-12' FROM dual UNION
  SELECT  DATE'2013-08-12' , NULL FROM dual UNION
   SELECT  DATE'2013-08-12',  DATE'2013-09-12' FROM dual)

   SELECT col1, col2 , substr(greatest('x'||col1,'x'||col2),2)
      FROM ABC;