例項：

SQL> select * from student;
        ID NAME                       DEPT
---------- -------------------- ----------


SQL> insert into student values(1,'小米',1);
SQL> insert into student values(2,'',2);
SQL> insert into student values(3,null,3);



SQL> select * from student  where  name like '%';
        ID NAME                       DEPT
---------- -------------------- ----------
         1 小米                          1

SQL> select * from student where name is null;
        ID NAME                       DEPT
---------- -------------------- ----------
         2                               2
         3                               3

SQL> select * from student where NVL(name,0) like '%';
        ID NAME                       DEPT
---------- -------------------- ----------
         1 小米                          1
         2                               2
         3                               3
 

2、當name為null時，NVL(name,0)的返回值為0，而0正好符合匹配LIKE '%'這個匹配模式，所以，name為null的資料行可以被查詢出來。