oracle sql filter for nullable values

Think of NULL as an unknown value and testing if something equals (or does not equal) an unknown will result in an unknown (NULL) as the answer. The SQL query will display results when the boolean filter is TRUE and this will not be the case if one value is NULL.

You can test the logic in PL/SQL (since it has an accessible BOOLEAN type):

SET SERVEROUTPUT ON;
DECLARE
  FUNCTION bool_to_string( bool BOOLEAN ) RETURN VARCHAR2
  AS
  BEGIN
    RETURN CASE WHEN bool IS NULL  THEN 'NULL'
                WHEN bool =  TRUE  THEN 'TRUE'
                WHEN bool =  FALSE THEN 'FALSE'
                ELSE                    'ERROR' END;
  END;
BEGIN
  DBMS_OUTPUT.PUT_LINE( 'A =  A    => ' || bool_to_string( 'A' = 'A' ) );
  DBMS_OUTPUT.PUT_LINE( 'A <> A    => ' || bool_to_string( 'A' <> 'A' ) );
  DBMS_OUTPUT.PUT_LINE( 'A =  NULL => ' || bool_to_string( 'A' = NULL ) );
  DBMS_OUTPUT.PUT_LINE( 'A <> NULL => ' || bool_to_string( 'A' <> NULL ) );
END;
/

Which outputs:

A =  A    => TRUE
A <> A    => FALSE
A =  NULL => NULL
A <> NULL => NULL

Note that the last two tests do not return FALSE but return NULL.

If you want to count including NULLs then you can do:

select count(*) from table where statuscode <> 13 OR statuscode IS NULL