SQL Query - How to remove null and replace with blank

Here is my query:

SELECT ename, sal, comm
 FROM EMP
 WHERE comm IS NULL
 ORDER BY sal DESC;
 

I would like to replace the NULL result in the comm column with a blank space. I have been told to under SELECT, write NULL(comm,' '), however this comes up with an error, saying invalid number. The query works if I have NULL(comm, 0), returning with 0 in the column.

From my understanding this means I can only have a number value in this column, however I want the blank space. Is someone able to please give me some guidance?

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