神秘的DUAL —— black_snail（原作）

標題 神秘的DUAL black_snail（原作）

關鍵字 ORACLE DUAL

DUAL ? 有什麼神秘的? 當你想得到ORACLE系統時間, 簡簡單單敲一行SQL

不就得了嗎? 故弄玄虛….

SQL> select sysdate from dual;

SYSDATE

---------

28-SEP-03

哈哈, 確實DUAL的使用很方便. 但是大家知道DUAL倒底是什麼OBJECT, 它有什麼特殊的行為嗎? 來,我們一起看一看.

首先搞清楚DUAL是什麼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

原來DUAL是屬於SYS schema的一個表,然後以PUBLIC SYNONYM的方式供其他資料庫USER使用.

再看看它的結構:

SQL> desc dual

Name Null? Type

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

DUMMY VARCHAR2(1)

SQL>

只有一個名字叫DUMMY的字元型COLUMN .

然後查詢一下表裡的資料:

SQL> select dummy from dual;

DUMMY

----------

X

哦, 只有一條記錄, DUMMY的值是’X’ .很正常啊,沒什麼奇怪嘛. 好,下面就有奇妙的東西出現了!

插入一條記錄:

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

迄今為止,一切正常. 然而當我們重新查詢記錄時,奇怪的事情發生了

SQL> select * from dual;

DUMMY

----------

X

剛才插入的那條記錄並沒有顯示出來 ! 明明DUAL表中有兩條記錄, 可就是只顯示一條!

再試一下刪除 ,狠一點,全刪光 !

SQL> delete from dual; /*注意沒有限定條件,試圖刪除全部記錄*/

1 row deleted.

SQL> commit;

Commit complete.

哈哈,也只有一條記錄被刪掉,

SQL> select * from dual;

DUMMY

----------

Y

為什麼會這樣呢? 難道SQL的文法對DUAL不起作用嗎?帶著這個疑問, 我查詢了一些ORACLE官方的資料. 原來ORACLE對DUAL表的操作做了一些內部處理,盡量保證DUAL表中只返回一條記錄.當然這寫內部操作是不可見的 .

看來ORACLE真是蘊藏著無窮的奧妙啊!

附: ORACLE關於DUAL表不同尋常特性的解釋

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