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

Use Oracle CASE... WHEN structure in your select:

SELECT COLA, COLB, CASE
  WHEN (COLA >= COLB OR COLB IS NULL)
    THEN COLA
  ELSE COLB
  END
  AS OUTPUT
FROM ...

Answer 2

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.

Answer 3

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),
    ...
)