Handling Null in Greatest function in Oracle

Your question specifically involves two columns, but I've run into situations where I needed GREATEST/LEAST of more than two columns. In those scenarios you can use COALESCE and expand the solution to as many columns you want.

Here is an example with three columns a, b, and c:

GREATEST(
    COALESCE(a, b, c),
    COALESCE(b, c, a),
    COALESCE(c, a, b)
)

Note that the column ordering of the COALESCE changes so that each input column is the first element COALESCE at least once. The only time this will return NULL is when all input columns are NULL.

In the "general solution" the number of COALESCE statements will be equal to the number of input columns:

GREATEST(
    COALESCE(col1, col2, col3, col4, ....),
    COALESCE(col2, col3, col4, ...., col1),
    COALESCE(col3, col4, ...., col1, col2),
    COALESCE(col4, ...., col1, col2, col3),
    COALESCE(...., col1, col2, col3, col4),
    ...
)

Another version using a case expression to handle the null values:

select cola, colb, 
  case when cola is null and colb is null then null
    when cola is null then colb
    when colb is null then cola
    else greatest(cola, colb)
  end as output
from <table>;

COLA       COLB       OUTPUT   
---------- ---------- ----------

09/21/2013 01/02/2012 09/21/2013 
           01/03/2013 01/03/2013 
01/03/2013            01/03/2013 

You might try the following:

SELECT cola, colb, COALESCE( GREATEST( cola, colb ), cola, colb ) AS output
  FROM yourtable;

The reason for COALESCE() is that GREATEST() returns NULL if either of the parameters is NULL.