��秘的DUAL

KingWell — Fri, 12 May 2006 02:17:00 GMT

�q�个是jrlee在帖子中转蝲的关于DUAL的东�?
看看�q�个可以对DUAL有个更深入的了解
引用�Q?em>原帖由�?jrlee" 发表�Q?br />
��秘的DUAL black_snail�Q�原作）(j��)

关键字 ORACLE DUAL

DUAL ? 有什么神�U�的? 当你惛_��到ORACLE�pȝ��旉��, ��单单敲一行SQL

不就得了�? 故弄玄虚�?

SQL>; select sysdate from dual;

SYSDATE

---------

28-SEP-03

哈哈, ��实DUAL的��用很方便. 但是大家知道DUAL倒底是什么OBJECT, 它有什么特�D�的行�ؓ(f��)�? �?我们一��L(f��ng)��一�?

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�?

SQL>; desc dual

Name Null? Type

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

DUMMY VARCHAR2(1)

SQL>;

只有一个名字叫DUMMY的字�W�型COLUMN .

然后查询一下表里的数据:

SQL>; select dummy from dual;

DUMMY

----------

X

�? 只有一条记�? DUMMY的值是’X’�?很正常啊,没什么奇怪嘛. �?下面��有奇妙的东西出��C��!

插入一条记�?

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

�q�今为止,一切正�? 然而当我们再次查询记录�?奇怪的事情发生了�?

SQL>; select * from dual;

DUMMY

----------

X

刚才插入的那条记录�ƈ没有昄��出来 ! 明明DUAL表中有两条记�? 可就是只昄��一�?

再试一下删除�?狠一�?全删光�?

SQL>; delete from dual; /*注意没有限定条�g,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

��Z��么会(x��)�q�样�? ��N��SQL的语法对DUAL不�v作用�?带着�q�个疑问, 我查询了一些ORACLE官方的资�? 原来ORACLE对DUAL表的操作做了一些内部处�?��量保证DUAL表中只返回一条记�?当然�q�写内部操作是不可见的�?

看来ORACLE真是蕴藏着无穷的奥妙啊!

�? 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

KingWell 2006-05-12 10:17 发表评论

oracle中的函数

KingWell — Fri, 12 May 2006 02:06:00 GMT

SQL中的单记录函�?br />1.ASCII
�q�回与指定的字符对应的十�q�制�?
SQL> select ascii('A') A,ascii('a') a,ascii('0') zero,ascii(' ') space from dual;

A A ZERO SPACE
--------- --------- --------- ---------
65 97 48 32

2.CHR
�l�出整数,�q�回对应的字�W?
SQL> select chr(54740) zhao,chr(65) chr65 from dual;

ZH C
-- -
�?A

3.CONCAT
�q�接两个字符�?
SQL> select concat('010-','88888888')||'�?3' 高乾竞电(sh��)�?from dual;

高乾竞电(sh��)�?br />----------------
010-88888888�?3

4.INITCAP
�q�回字符串�ƈ��字�W�串的第一个字母变为大�?
SQL> select initcap('smith') upp from dual;

UPP
-----
Smith

5.INSTR(C1,C2,I,J)
在一个字�W�串中搜索指定的字符,�q�回发现指定的字�W�的位置;
C1    被搜索的字符�?br />C2    希望搜烦(ch��)的字�W�串
I     搜烦(ch��)的开始位�|?默认�?
J     出现的位�|?默认�?
SQL> select instr('oracle traning','ra',1,2) instring from dual;

INSTRING
---------
9

6.LENGTH
�q�回字符串的长度;
SQL> select name,length(name),addr,length(addr),sal,length(to_char(sal)) from gao.nchar_tst;

NAME LENGTH(NAME) ADDR LENGTH(ADDR) SAL LENGTH(TO_CHAR(SAL))
------ ------------ ---------------- ------------ --------- --------------------
高乾竞 �?3 北京市�v锭区 6 9999.99 7

7.LOWER
�q�回字符�?�q�将所有的字符��写
SQL> select lower('AaBbCcDd')AaBbCcDd from dual;

AABBCCDD
--------
aabbccdd

8.UPPER
�q�回字符�?�q�将所有的字符大写
SQL> select upper('AaBbCcDd') upper from dual;

UPPER
--------
AABBCCDD

9.RPAD和LPAD(�_�脓(chu��ng)字符)
RPAD 在列的右边粘贴字�W?br />LPAD 在列的左边粘贴字�W?br />SQL> select lpad(rpad('gao',10,'*'),17,'*')from dual;

LPAD(RPAD('GAO',1
-----------------
*******gao*******
不够字符则用*来填�?/font>

10.LTRIM和RTRIM
LTRIM 删除左边出现的字�W�串
RTRIM 删除双��出现的字�W�串
SQL> select ltrim(rtrim(' gao qian jing ',' '),' ') from dual;

LTRIM(RTRIM('
-------------
gao qian jing

11.SUBSTR(string,start,count)
取子字符�?从start开�?取count�?br />SQL> select substr('13088888888',3,8) from dual;

SUBSTR('
--------
08888888

12.REPLACE('string','s1','s2')
string 希望被替换的字符或变�?
s1 被替换的字符�?br />s2 要替换的字符�?br />SQL> select replace('he love you','he','i') from dual;

REPLACE('H
----------
i love you

13.SOUNDEX
�q�回一个与�l�定的字�W�串读音相同的字�W�串
SQL> create table table1(xm varchar(8));
SQL> insert into table1 values('weather');
SQL> insert into table1 values('wether');
SQL> insert into table1 values('gao');

SQL> select xm from table1 where soundex(xm)=soundex('weather');

XM
--------
weather
wether

14.TRIM('s' from 'string')
LEADING 剪掉前面的字�W?br />TRAILING 剪掉后面的字�W?br />如果不指�?默认为空格符

15.ABS
�q�回指定值的�l�对�?br />SQL> select abs(100),abs(-100) from dual;

ABS(100) ABS(-100)
--------- ---------
100 100

16.ACOS
�l�出反余弦的�?br />SQL> select acos(-1) from dual;

ACOS(-1)
---------
3.1415927

17.ASIN
�l�出反正弦的�?br />SQL> select asin(0.5) from dual;

ASIN(0.5)
---------
.52359878

18.ATAN
�q�回一个数字的反正切�?br />SQL> select atan(1) from dual;

ATAN(1)
---------
.78539816

19.CEIL
�q�回大于或等于给出数字的最��整�?br />SQL> select ceil(3.1415927) from dual;

CEIL(3.1415927)
---------------
4

20.COS
�q�回一个给定数字的余��u
SQL> select cos(-3.1415927) from dual;

COS(-3.1415927)
---------------
-1

21.COSH
�q�回一个数字反余��u�?br />SQL> select cosh(20) from dual;

COSH(20)
---------
242582598

22.EXP
�q�回一个数字e的n�ơ方�?br />SQL> select exp(2),exp(1) from dual;

EXP(2) EXP(1)
--------- ---------
7.3890561 2.7182818

23.FLOOR
对给定的数字取整�?br />SQL> select floor(2345.67) from dual;

FLOOR(2345.67)
--------------
2345

24.LN
�q�回一个数字的�Ҏ(gu��)��?br />SQL> select ln(1),ln(2),ln(2.7182818) from dual;

LN(1) LN(2) LN(2.7182818)
--------- --------- -------------
0 .69314718 .99999999

25.LOG(n1,n2)
�q�回一个以n1为底n2的对�?
SQL> select log(2,1),log(2,4) from dual;

LOG(2,1) LOG(2,4)
--------- ---------
0 2

26.MOD(n1,n2)
�q�回一个n1除以n2的余�?br />SQL> select mod(10,3),mod(3,3),mod(2,3) from dual;

MOD(10,3) MOD(3,3) MOD(2,3)
--------- --------- ---------
1 0 2

27.POWER
�q�回n1的n2�ơ方�?br />SQL> select power(2,10),power(3,3) from dual;

POWER(2,10) POWER(3,3)
----------- ----------
1024 27

28.ROUND和TRUNC
按照指定的精度进行舍�?br />SQL> select round(55.5),round(-55.4),trunc(55.5),trunc(-55.5) from dual;

ROUND(55.5) ROUND(-55.4) TRUNC(55.5) TRUNC(-55.5)
----------- ------------ ----------- ------------
56 -55 55 -55

29.SIGN
取数字n的符�?大于0�q�回1,��于0�q�回-1,�{�于0�q�回0
SQL> select sign(123),sign(-100),sign(0) from dual;

SIGN(123) SIGN(-100) SIGN(0)
--------- ---------- ---------
1 -1 0

30.SIN
�q�回一个数字的正��u�?br />SQL> select sin(1.57079) from dual;

SIN(1.57079)
------------
1

31.SIGH
�q�回双曲正��u的�?br />SQL> select sin(20),sinh(20) from dual;

SIN(20) SINH(20)
--------- ---------
.91294525 242582598

32.SQRT
�q�回数字n的根
SQL> select sqrt(64),sqrt(10) from dual;

SQRT(64) SQRT(10)
--------- ---------
8 3.1622777

33.TAN
�q�回数字的正切�?br />SQL> select tan(20),tan(10) from dual;

TAN(20) TAN(10)
--------- ---------
2.2371609 .64836083

34.TANH
�q�回数字n的双曲正切�?br />SQL> select tanh(20),tan(20) from dual;

TANH(20) TAN(20)
--------- ---------
1 2.2371609

35.TRUNC
按照指定的精度截取一个数
SQL> select trunc(124.1666,-2) trunc1,trunc(124.16666,2) from dual;

TRUNC1 TRUNC(124.16666,2)
--------- ------------------
100 124.16

36.ADD_MONTHS
增加或减��L��?br />SQL> select to_char(add_months(to_date('199912','yyyymm'),2),'yyyymm') from dual;

TO_CHA
------
200002
SQL> select to_char(add_months(to_date('199912','yyyymm'),-2),'yyyymm') from dual;

TO_CHA
------
199910

37.LAST_DAY
�q�回日期的最后一�?br />SQL> select to_char(sysdate,'yyyy.mm.dd'),to_char((sysdate)+1,'yyyy.mm.dd') from dual;

TO_CHAR(SY TO_CHAR((S
---------- ----------
2004.05.09 2004.05.10
SQL> select last_day(sysdate) from dual;

LAST_DAY(S
----------
31-5�?-04

38.MONTHS_BETWEEN(date2,date1)
�l�出date2-date1的月�?br />SQL> select months_between('19-12�?1999','19-3�?1999') mon_between from dual;

MON_BETWEEN
-----------
9
SQL>selectmonths_between(to_date('2000.05.20','yyyy.mm.dd'),to_date('2005.05.20','yyyy.mm.dd')) mon_betw from dual;

MON_BETW
---------
-60

39.NEW_TIME(date,'this','that')
�l�出在this时区=other时区的日期和旉��
SQL> select to_char(sysdate,'yyyy.mm.dd hh24:mi:ss') bj_time,to_char(new_time
2 (sysdate,'PDT','GMT'),'yyyy.mm.dd hh24:mi:ss') los_angles from dual;

BJ_TIME LOS_ANGLES
------------------- -------------------
2004.05.09 11:05:32 2004.05.09 18:05:32

40.NEXT_DAY(date,'day')
�l�出日期date和星期x之后计算下一个星期的日期
SQL> select next_day('18-5�?2001','星期�?) next_day from dual;

NEXT_DAY
----------
25-5�?-01

41.SYSDATE
用来得到�pȝ��的当前日�?br />SQL> select to_char(sysdate,'dd-mm-yyyy day') from dual;

TO_CHAR(SYSDATE,'
-----------------
09-05-2004 星期�?br />trunc(date,fmt)按照�l�出的要求将日期截断,如果fmt='mi'表示保留�?截断�U?br />SQL> select to_char(trunc(sysdate,'hh'),'yyyy.mm.dd hh24:mi:ss') hh,
2 to_char(trunc(sysdate,'mi'),'yyyy.mm.dd hh24:mi:ss') hhmm from dual;

HH HHMM
------------------- -------------------
2004.05.09 11:00:00 2004.05.09 11:17:00

42.CHARTOROWID
��字�W�数据类型�{换�ؓ(f��)ROWID�c�d��
SQL> select rowid,rowidtochar(rowid),ename from scott.emp;

ROWID ROWIDTOCHAR(ROWID) ENAME
------------------ ------------------ ----------
AAAAfKAACAAAAEqAAA AAAAfKAACAAAAEqAAA SMITH
AAAAfKAACAAAAEqAAB AAAAfKAACAAAAEqAAB ALLEN
AAAAfKAACAAAAEqAAC AAAAfKAACAAAAEqAAC WARD
AAAAfKAACAAAAEqAAD AAAAfKAACAAAAEqAAD JONES

43.CONVERT(c,dset,sset)
��源字符�?sset从一个语�a�字符集�{换到另一个目的dset字符�?br />SQL> select convert('strutz','we8hp','f7dec') "conversion" from dual;

conver
------
strutz

44.HEXTORAW
��一个十六进制构成的字符串�{换�ؓ(f��)二进�?/font>

45.RAWTOHEXT
��一个二�q�制构成的字�W�串转换为十六进�?/font>

46.ROWIDTOCHAR
��ROWID数据�c�d��转换为字�W�类�?/font>

47.TO_CHAR(date,'format')
SQL> select to_char(sysdate,'yyyy/mm/dd hh24:mi:ss') from dual;

TO_CHAR(SYSDATE,'YY
-------------------
2004/05/09 21:14:41

48.TO_DATE(string,'format')
��字�W�串转化为ORACLE中的一个日�?/font>

49.TO_MULTI_BYTE
��字�W�串中的单字节字�W��{化�ؓ(f��)多字节字�W?br />SQL> select to_multi_byte('�?) from dual;

TO
--
�?/font>

50.TO_NUMBER
��给出的字符转换为数�?br />SQL> select to_number('1999') year from dual;

YEAR
---------
1999

51.BFILENAME(dir,file)
指定一个外部二�q�制文�g
SQL>insert into file_tb1 values(bfilename('lob_dir1','image1.gif'));

52.CONVERT('x','desc','source')
��x字段或变量的源source转换为desc
SQL> select sid,serial#,username,decode(command,
2 0,'none',
3 2,'insert',
4 3,
5 'select',
6 6,'update',
7 7,'delete',
8 8,'drop',
9 'other') cmd from v$session where type!='background';

      SID   SERIAL# USERNAME                       CMD
--------- --------- ------------------------------ ------
        1         1                                none
        2         1                                none
        3         1                                none
        4         1                                none
        5         1                                none
        6         1                                none
        7      1275                                none
        8      1275                                none
        9        20 GAO                            select
       10        40 GAO                            none

53.DUMP(s,fmt,start,length)
DUMP函数以fmt指定的内部数字格式返回一个VARCHAR2�c�d��的�?br />SQL> col global_name for a30
SQL> col dump_string for a50
SQL> set lin 200
SQL> select global_name,dump(global_name,1017,8,5) dump_string from global_name;

GLOBAL_NAME DUMP_STRING
------------------------------ --------------------------------------------------
ORACLE.WORLD Typ=1 Len=12 CharacterSet=ZHS16GBK: W,O,R,L,D

54.EMPTY_BLOB()和EMPTY_CLOB()
�q�两个函数都是用来对大数据类型字�D�进行初始化操作的函�?/font>

55.GREATEST
�q�回一�l�表辑ּ�中的最大�?��x��较字�W�的�~�码大小.
SQL> select greatest('AA','AB','AC') from dual;

GR
--
AC
SQL> select greatest('�?,'�?,'�?) from dual;

GR
--
�?/font>

56.LEAST
�q�回一�l�表辑ּ�中的最��?
SQL> select least('�?,'�?,'�?) from dual;

LE
--
�?/font>

57.UID
�q�回标识当前用户的唯一整数
SQL> show user
USER �?GAO"
SQL> select username,user_id from dba_users where user_id=uid;

USERNAME USER_ID
------------------------------ ---------
GAO 25

58.USER
�q�回当前用户的名�?br />SQL> select user from dual;

USER
------------------------------
GAO

59.USEREVN
�q�回当前用户环境的信�?opt可以�?
ENTRYID,SESSIONID,TERMINAL,ISDBA,LABLE,LANGUAGE,CLIENT_INFO,LANG,VSIZE
ISDBA 查看当前用户是否是DBA如果是则�q�回true
SQL> select userenv('isdba') from dual;

USEREN
------
FALSE
SQL> select userenv('isdba') from dual;

USEREN
------
TRUE
SESSION
�q�回�?x��)话标�?br />SQL> select userenv('sessionid') from dual;

USERENV('SESSIONID')
--------------------
152
ENTRYID
�q�回�?x��)话人口标�?br />SQL> select userenv('entryid') from dual;

USERENV('ENTRYID')
------------------
0
INSTANCE
�q�回当前INSTANCE的标�?br />SQL> select userenv('instance') from dual;

USERENV('INSTANCE')
-------------------
1
LANGUAGE
�q�回当前环境变量
SQL> select userenv('language') from dual;

USERENV('LANGUAGE')
----------------------------------------------------
SIMPLIFIED CHINESE_CHINA.ZHS16GBK
LANG
�q�回当前环境的语�a�的羃�?br />SQL> select userenv('lang') from dual;

USERENV('LANG')
----------------------------------------------------
ZHS
TERMINAL
�q�回用户的终端或机器的标�?br />SQL> select userenv('terminal') from dual;

USERENV('TERMINA
----------------
GAO
VSIZE(X)
�q�回X的大��?字节)�?br />SQL> select vsize(user),user from dual;

VSIZE(USER) USER
----------- ------------------------------
6 SYSTEM

60.AVG(DISTINCT|ALL)
all表示�Ҏ(gu��)��有的值求�q�_��?distinct只对不同的值求�q�_��?br />SQLWKS> create table table3(xm varchar(8),sal number(7,2));
语句已处理�?br />SQLWKS> insert into table3 values('gao',1111.11);
SQLWKS> insert into table3 values('gao',1111.11);
SQLWKS> insert into table3 values('zhu',5555.55);
SQLWKS> commit;

SQL> select avg(distinct sal) from gao.table3;

AVG(DISTINCTSAL)
----------------
3333.33

SQL> select avg(all sal) from gao.table3;

AVG(ALLSAL)
-----------
2592.59

61.MAX(DISTINCT|ALL)
求最大�?ALL表示�Ҏ(gu��)��有的值求最大�?DISTINCT表示对不同的值求最大�?相同的只取一��?br />SQL> select max(distinct sal) from scott.emp;

MAX(DISTINCTSAL)
----------------
5000

62.MIN(DISTINCT|ALL)
求最��?ALL表示�Ҏ(gu��)��有的值求最��?DISTINCT表示对不同的值求最��?相同的只取一��?br />SQL> select min(all sal) from gao.table3;

MIN(ALLSAL)
-----------
1111.11

63.STDDEV(distinct|all)
求标准差,ALL表示�Ҏ(gu��)��有的值求标准�?DISTINCT表示只对不同的值求标准�?br />SQL> select stddev(sal) from scott.emp;

STDDEV(SAL)
-----------
1182.5032

SQL> select stddev(distinct sal) from scott.emp;

STDDEV(DISTINCTSAL)
-------------------
1229.951

64.VARIANCE(DISTINCT|ALL)
求协方差

SQL> select variance(sal) from scott.emp;

VARIANCE(SAL)
-------------
1398313.9

65.GROUP BY
主要用来对一�l�数�q�行�l�计
SQL> select deptno,count(*),sum(sal) from scott.emp group by deptno;

   DEPTNO COUNT(*) SUM(SAL)
--------- --------- ---------
       10         3      8750
       20         5     10875
       30         6      9400

66.HAVING
对分�l�统计再加限制条�?br />SQL> select deptno,count(*),sum(sal) from scott.emp group by deptno having count(*)>=5;

   DEPTNO COUNT(*) SUM(SAL)
--------- --------- ---------
       20         5     10875
       30         6      9400
SQL> select deptno,count(*),sum(sal) from scott.emp having count(*)>=5 group by deptno ;

   DEPTNO COUNT(*) SUM(SAL)
--------- --------- ---------
       20         5     10875
       30         6      9400

67.ORDER BY
用于�Ҏ(gu��)��询到的结果进行排序输�?br />SQL> select deptno,ename,sal from scott.emp order by deptno,sal desc;

   DEPTNO ENAME            SAL
--------- ---------- ---------
       10 KING            5000
       10 CLARK           2450
       10 MILLER          1300
       20 SCOTT           3000
       20 FORD            3000
       20 JONES           2975
       20 ADAMS           1100
       20 SMITH            800
       30 BLAKE           2850
       30 ALLEN           1600
       30 TURNER          1500
       30 WARD            1250
       30 MARTIN          1250
       30 JAMES            950

作者Blog�Q?/strong> http://blog.csdn.net/AustinLei/

KingWell 2006-05-12 10:06 发表评论

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���秘的DUAL

oracle中的函数

��秘的DUAL