When/Why does Oracle adds NaN to a row in a database table

Question

I know that NaN stands for Not a Number. But, I have trouble understanding when and why Oracle adds this to a row.

Is it when it encounters a value less than 0 like a negative number or when its a garbage value.

Answer 1

From the documentaton:

The Oracle Database numeric data types store positive and negative fixed and floating-point numbers, zero, infinity, and values that are the undefined result of an operation—"not a number" or NAN.

As far as I'm aware you can only get NaN in a binary_float or binary_double column; those data types have their own literals for NaN too, and there's an is nan condition for them too, and the nanvl() function to manipulate them.

An example of a way to get such a value is to divide a zero float/double value by zero:

select 0f/0 from dual;

0F/0
----
NaN  

... so if you're seeing NaNs your application logic or underlying data might be broken. (Note you can't get this with a 'normal' number type; you get ORA-01476: divisor is equal to zero unless the numerator is float or double).

You won't get NaN for zero or negative numbers though. It's also possible you have a string column and an application is putting the word 'NaN' in, but storing numbers as strings is a bad idea on many levels, so hopefully that is not the case.

Answer 2

Nope <=0 is still a number so not quite. NaN (or infinity) are special values that the DB uses to keep it's sanity when dealing with non-computable numbers (+-∞, or simply something that is not a number). Here's some code:

DECLARE
  l_bd_test   binary_double;
  l_int_test  INTEGER;
BEGIN
  l_bd_test   := 'NAN';
  l_int_test  := 0;
  IF l_bd_test IS NAN THEN 
    DBMS_OUTPUT.PUT_LINE(l_bd_test || ' IS NAN');
  ELSE
    DBMS_OUTPUT.PUT_LINE(l_bd_test || ' IS A #');
  END IF;
  IF l_int_test IS NAN THEN 
    DBMS_OUTPUT.PUT_LINE(l_int_test || ' IS NAN');
  ELSE
    DBMS_OUTPUT.PUT_LINE(l_int_test || ' IS A #');
  END IF;
END;
/

Substitute NAN for INFINITY or even negate it and see the results.