GREATEST is not to be used on a singular item. Although it will return values with single item, as input the effect is not useful. Use GREATEST when you
need to get the greatest values from a list of values. MAX on the other hand will return the max value on a column.
Here is what the documentation gives :
GREATEST returns the greatest of the list of exprs. All exprs. after the first are implicitly converted to the datatype of the first expr. before the comparison. Oracle compares the exprs. using nonpadded comparison
semantics. Character comparison is based on the value of the character in the database character set. One character is greater than another if it has a higher
character set value. If the value returned by this function is character data, its datatype is always VARCHAR2.
SELECT GREATEST ('HARRY', 'HARRIOT', 'HAROLD')
"Greatest" FROM DUAL;
MAX returns maximum value of expr. You can use it as an aggregate or analytic function. If you specify DISTINCT, you can specify only the query_partition_clause of the analytic_clause. The ORDER_BY_clause and windowing_clause are not allowed.