ORACLE的DUAL表

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

 

dual是一个虚拟表，用来构成select的语法规则，oracle保证dual里面永远只有一条记录。我们可以用它来做很多事情，如下：

1、查看当前用户，可以在 SQL Plus中执行下面语句

select user from dual;

2、用来调用系统函数

select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual;--获得当前系统时间

select SYS_CONTEXT('USERENV','TERMINAL') from dual;--获得主机名

select SYS_CONTEXT('USERENV','language') from dual;--获得当前locale

select dbms_random.random from dual;--获得一个随机数

2.1 查找当天日期

SQL> select sysdate from dual;

SYSDATE
-----------
2004-4-13

2.2 查找当前日期的当月第一天

   SQL> select trunc(sysdate,\'MONTH\') from dual;

TRUNC(SYSDATE,\'MONTH\')
----------------------
2004-4-1

2.3 查找当前日期的当月最后一天

SQL> select trunc(last_day(sysdate)) from dual;

TRUNC(LAST_DAY(SYSDATE))
------------------------
2004-4-30

3、得到序列的下一个值或当前值，用下面语句

select your_sequence.nextval from dual;--获得序列your_sequence的下一个值

select your_sequence.currval from dual;--获得序列your_sequence的当前值

4、可以用做计算器

select 7*9 from dual;