Oracle之NULL IS NULL

NULL 不 大于/小于/等于/不等于 任何值（包括NULL本身），有且仅有：NULL IS NULL

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

原文链接： http://blog.csdn.net/t0nsha/article/details/6435328