神秘的DUAL

DUAL ? 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