Oracle之NULL IS NULL

NULL 不 大於/小於/等於/不等於 任何值（包括NULL本身），有且僅有：NULL IS NULL

SQL> set serveroutput on<br />SQL><br />SQL> BEGIN<br /> 2 IF (2 > NULL)<br /> 3 THEN<br /> 4 dbms_output.put_line('>');<br /> 5 ELSIF (2 < NULL)<br /> 6 THEN<br /> 7 dbms_output.put_line('<');<br /> 8 ELSIF (2 = NULL)<br /> 9 THEN<br /> 10 dbms_output.put_line('=');<br /> 11 ELSIF (2 != NULL)<br /> 12 THEN<br /> 13 dbms_output.put_line('!=');<br /> 14 ELSE<br /> 15 dbms_output.put_line('Unknown!');<br /> 16 END IF;<br /> 17<br /> 18 IF (NULL = NULL)<br /> 19 THEN<br /> 20 dbms_output.put_line('NULL = NULL');<br /> 21 ELSIF (NULL != NULL)<br /> 22 THEN<br /> 23 dbms_output.put_line('NULL != NULL');<br /> 24 ELSIF (NULL IS NULL)<br /> 25 THEN<br /> 26 dbms_output.put_line('NULL IS NULL');<br /> 27 ELSE<br /> 28 dbms_output.put_line('UNKNOWN!!');<br /> 29 END IF;<br /> 30 END;<br /> 31 /</p><p>Unknown!<br />NULL IS NULL</p><p>PL/SQL procedure successfully completed</p><p>SQL>