神秘的DUAL

DUAL ? MILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">有什么神秘的? 当你想得到ORACLE系统时间, 简简单单敲一行SQL

不就得了吗? 故弄玄虚….

SQL> select sysdate from dual;

SYSDATE

---------

28-SEP-03

哈哈, 确实DUAL的使用很方便. 但是大家知道DUAL倒底是什么OBJECT, 它有什么特殊的行为吗? 来,我们一起看一看.

首先搞清楚DUAL是什么OBJECT :

SQL> connect system/manager

Connected.

SQL> select owner, object_name , object_type from dba_objects where object_name like '%DUAL%';

OWNER           OBJECT_NAME     OBJECT_TYPE

---------------        ---------------          -------------

SYS              DUAL               TABLE

PUBLIC           DUAL              SYNONYM

原来DUAL是属于SYS schema的一个表,然后以PUBLIC SYNONYM的方式供其他数据库USER使用.

再看看它的结构:

SQL> desc dual

Name                        Null?  Type

 ----------------------------------------- -------- ----------------------------

DUMMY                           VARCHAR2(1)

SQL>

只有一个名字叫DUMMY的字符型COLUMN .

            然后查询一下表里的数据:

SQL> select dummy from dual;

DUMMY

----------

X

哦, 只有一条记录, DUMMY的值是’X’ .很正常啊,没什么奇怪嘛. 好,下面就有奇妙的东西出现了!

插入一条记录:

SQL> connect sys as sysdba

Connected.

SQL> insert into dual values ( 'Y');

1 row created.

SQL> commit;

Commit complete.

SQL> select count(*) from dual;

COUNT(*)

----------

      2

迄今为止,一切正常. 然而当我们再次查询记录时,奇怪的事情发生了

SQL> select * from dual;

DUMMY

----------

X

刚才插入的那条记录并没有显示出来 ! 明明DUAL表中有两条记录, 可就是只显示一条!

再试一下删除 ,狠一点,全删光 !

SQL> delete from dual;  /*注意没有限定条件,试图删除全部记录*/

1 row deleted.

SQL> commit;

Commit complete.

哈哈,也只有一条记录被删掉,

SQL> select * from dual;

DUMMY

----------

Y

为什么会这样呢? 难道SQL的语法对DUAL不起作用吗?带着这个疑问, 我查询了一些ORACLE官方的资料. 原来ORACLE对DUAL表的操作做了一些内部处理,尽量保证DUAL表中只返回一条记录.当然这写内部操作是不可见的 .

看来ORACLE真是蕴藏着无穷的奥妙啊!

附: ORACLE关于DUAL表不同寻常特性的解释

There is internalized code that makes this happen.  Code checks that ensure 

that a table scan of SYS.DUAL only returns one row.  Svrmgrl behaviour is 

incorrect but this is now an obsolete product.

The base issue you should always remember and keep is:  DUAL table should always

have 1 ROW.  Dual is a normal table with one dummy column of varchar2(1).

This is basically used from several applications as a pseudo table for

getting results from a select statement that use functions like sysdate or other

prebuilt or application functions.  If DUAL has no rows at all some applications

(that use DUAL) may fail with NO_DATA_FOUND exception.  If DUAL has more than 1

row then applications (that use DUAL) may fail with TOO_MANY_ROWS exception.  

So DUAL should ALWAYS have 1 and only 1 row