引用Q?em>原帖由?jrlee" 发表Q?br />
秘的DUAL black_snailQ原作）
关键字 ORACLE DUAL
DUAL ? 有什么神U的? 当你惛_到ORACLEpȝ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的行ؓ? ?我们一L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q样? NSQL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
]]>
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?from dual;
高乾竞电?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 希望搜烦的字W串
I 搜烦的开始位|?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nơ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?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n2ơ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0q回1,于0q回-1,{于0q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{换ؓROWIDcd
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{换ؓ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cd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{化ؓ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VARCHAR2cd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回会话标志
SQL> select userenv('sessionid') from dual;
USERENV('SESSIONID')
--------------------
152
ENTRYID
q回会话人口标志
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
]]>