ORACLE 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
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(假的, 伪的, 冒充的) 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 
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