Special NULL value 

A special NULL value is a special data type and is the result of arithmetic operations that lead to an overflow or a division by 0.

The special NULL value is only permitted for output columns and for columns in the ORDER clause. If an overflow occurs in an arithmetic operation or a division by 0 at another point, the SQL statement is abnormally terminated.

The comparison of a special NULL value with any value is always undefined.

As far as sorting is concerned, the special NULL value is greater than all non-NULL values, but less than the NULL value.