系统定义的表,只有一笔记录.可以用来返回函数值. select user from dual;返回当前用户.
虚表（dual）是oracle提供的最小的工作表，它仅包含一行一列。对于虚表（dual）来说，其中的列往往是不相关的或无关紧要的。 如：查询当前的系统日期 SQL> select sysdate from dual;SYSDATE ------------------- 2004/04/28 08:49:41
给你最详细的： 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/managerConnected.SQL> select owner, object_name , object_type from dba_objects where object_name like '%DUAL%'; OWNER OBJECT_NAME OBJECT_TYPE--------------- --------------- -------------SYS DUAL TABLEPUBLIC 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 dualName 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 sysdbaConnected.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
select user from dual;返回当前用户.
如：查询当前的系统日期
SQL> select sysdate from dual;SYSDATE
-------------------
2004/04/28 08:49:41
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/managerConnected.SQL> select owner, object_name , object_type from dba_objects where object_name like '%DUAL%'; OWNER OBJECT_NAME OBJECT_TYPE--------------- --------------- -------------SYS DUAL TABLEPUBLIC 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 dualName 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 sysdbaConnected.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