神秘的DUAL
2024-07-21 02:08:38
供稿：網友
dual ? 有什么神秘的? 當你想得到oracle系統(tǒng)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倒底是什么object, 它有什么特殊的行為嗎? 來,我們一起看一看.
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
原來dual是屬于sys schema的一個表,然后以public synonym的方式供其他數據庫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的字符型column .
然后查詢一下表里的數據:
sql> select dummy from dual;
dummy
----------
x
哦, 只有一條記錄, dummy的值是’x’ .很正常啊,沒什么奇怪嘛. 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. 然而當我們再次查詢記錄時,奇怪的事情發(fā)生了
sql> select * from dual;
dummy
----------
x
剛才插入的那條記錄并沒有顯示出來 ! 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
為什么會這樣呢? 難道sql的語法對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關于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
