DBMS_RADOM——Oracle中的随机函数
最近闲着没事，又看了一下PLSQL的Package，今天来学习一下DBMS_RADOM包，简单了解Oracle取随机函数的方法。最后还附上在Oracle中另一种可以取到随机数的方法。OK，首先按照惯例，先来看一下dbms_random包在Oracle中自带的注释说明：
*****************************************************************************
CREATE OR REPLACE PACKAGE SYS.dbms_random AS
------------
-- OVERVIEW
--
-- This package should be installed as SYS. It generates a sequence of
-- random 38-digit Oracle numbers. The expected length of the sequence
-- is about power(10,28), which is hopefully long enough.
--
--------
-- USAGE
--
-- This is a random number generator. Do not use for cryptography.
-- For more options the cryptographic toolkit should be used.
--
-- By default, the package is initialized with the current user
-- name, current time down to the second, and the current session.
--
-- If this package is seeded twice with the same seed, then accessed
-- in the same way, it will produce the same results in both cases.
--
--------
-- EXAMPLES
--
-- To initialize or reset the generator, call the seed procedure as in:
-- execute dbms_random.seed(12345678);
-- or
-- execute dbms_random.seed(TO_CHAR(SYSDATE,'MM-DD-YYYY HH24:MI:SS'));
-- To get the random number, simply call the function, e.g.
-- my_random_number BINARY_INTEGER;
-- my_random_number := dbms_random.random;
-- or
-- my_random_real NUMBER;
-- my_random_real := dbms_random.value;
-- To use in SQL statements:
-- select dbms_random.value from dual;
-- insert into a values (dbms_random.value);
-- variable x NUMBER;
-- execute :x := dbms_random.value;
-- update a set a2=a2+1 where a1 < :x;
-- Seed with a binary integer
PROCEDURE seed(val IN BINARY_INTEGER);
PRAGMA restrict_references (seed, WNDS);
-- Seed with a string (up to length 2000)
PROCEDURE seed(val IN VARCHAR2);
PRAGMA restrict_references (seed, WNDS);
-- Get a random 38-digit precision number, 0.0 <= value < 1.0
FUNCTION value RETURN NUMBER;
PRAGMA restrict_references (value, WNDS);
-- get a random Oracle number x, low <= x < high
FUNCTION value (low IN NUMBER, high IN NUMBER) RETURN NUMBER;
PRAGMA restrict_references (value, WNDS);
-- get a random number from a normal distribution
FUNCTION normal RETURN NUMBER;
PRAGMA restrict_references (normal, WNDS);
-- get a random string
FUNCTION string (opt char, len NUMBER)
/* "opt" specifies that the returned string may contain:
'u','U' : upper case alpha characters only
'l','L' : lower case alpha characters only
'a','A' : alpha characters only (mixed case)
'x','X' : any alpha-numeric characters (upper)
'p','P' : any printable characters
*/
RETURN VARCHAR2; -- string of <len> characters (max 60)
PRAGMA restrict_references (string, WNDS);
-- Obsolete, just calls seed(val)
PROCEDURE initialize(val IN BINARY_INTEGER);
PRAGMA restrict_references (initialize, WNDS);
-- Obsolete, get integer in ( -power(2,31) <= random < power(2,31) )
FUNCTION random RETURN BINARY_INTEGER;
PRAGMA restrict_references (random, WNDS);
-- Obsolete, does nothing
PROCEDURE terminate;
TYPE num_array IS TABLE OF NUMBER INDEX BY BINARY_INTEGER;
END dbms_random;
*****************************************************************************
简单得说，通过dbms_random包调用随机数的方法大致有4种：
1、dbms_random.normal
这个函数不带参数，能返回normal distribution的一个number类型，所以基本上随机数会在-1到1之间。
简单测试了一下，产生100000次最大能到5左右：
SQL> declare
2 i number:=0;
3 j number:=0;
4 begin
5 for k in 1 .. 100000 loop
6 i:= dbms_random.normal;
7 if i > j
8 then j:=i;
9 end if;
10 end loop;
11 dbms_output.put_line(j);
12 end;
13 /
5.15325081797418404136433867107468983182
PL/SQL procedure successfully completed
2、dbms_random.random
这个也没有参数，返回一个从-power(2,31)到power(2,31)的整数值
3、dbms_random.value
这个函数分为两种，一种是没有参数，则直接返回0-1之间的38位小数
SQL > column value format 9.99999999999999999999999999999999999999
SQL > select dbms_random.value from dual;
VALUE
-----------------------------------------
.58983014999643548701631750396301271752
第二种是加上两个参数a、b，则返回值在a、b之间的38位小数
SQL > column value format 999.999999999999999999999999999999999999
SQL > select dbms_random.value(100,500) value from dual;
VALUE
-----------------------------------------
412.150194612502916808701157054098274240
注意：无论前面几位，小数点之后都是38位
4、dbms_random.string
这个函数必须带有两个参数，前面的字符指定类型，后面的数值指定位数(最大60)
类型说明：
'u','U' : upper case alpha characters only
'l','L' : lower case alpha characters only
'a','A' : alpha characters only (mixed case)
'x','X' : any alpha-numeric characters (upper)
'p','P' : any printable characters
SQL > column value format a30
SQL > select dbms_random.string('u',30) value from dual;
VALUE
------------------------------
VTQNLGISELPXEDBXKUZLXKBAJMUTIA
SQL > select dbms_random.string('l',30) value from dual;
VALUE
------------------------------
uqygsbquingfqdytpgjvdoblxeglgu
SQL > select dbms_random.string('a',30) value from dual;
VALUE
------------------------------
NGTGkQypuSWhBfcrHiOlQwOUXkqJjy
SQL > select dbms_random.string('x',30) value from dual;
VALUE
------------------------------
UVWONYJMXT31VEFPD736WJCJ5QT6BD
SQL > select dbms_random.string('p',30) value from dual;
VALUE
------------------------------
:mak$(WT4M_7c/+f[_XUscf$P Zcq{
5、关于seed
可以设置seed来确定随机数的起始点，对于相同的seed而言，随机数的任意一次变化都将是确定的。
就是说，如果在某一时刻调用了seed，之后第一次产生的随机数是4，第二次是6，第三次是1，那么当你再次调用相同的seed之后，一次产生的随机数还是4、6、1
seed有两种，一种是数值型的，一种是字符型(最大长度2000)的
-- Seed with a binary integer
PROCEDURE seed(val IN BINARY_INTEGER);
PRAGMA restrict_references (seed, WNDS);
-- Seed with a string (up to length 2000)
PROCEDURE seed(val IN VARCHAR2);
PRAGMA restrict_references (seed, WNDS);
6、关于initialize
一个integer参数，注释说的很清楚了：
-- Obsolete, just calls seed(val)
PROCEDURE initialize(val IN BINARY_INTEGER);
PRAGMA restrict_references (initialize, WNDS);
sys_guid()
官方文档的说明如下：
SYS_GUID generates and returns a globally unique identifier (RAW value) made up of 16 bytes. On most platforms, the generated identifier consists of a host identifier, a process or thread identifier of the process or thread invoking the function, and a nonrepeating value (sequence of bytes) for that process or thread.
简单得说就是，随机生成一个32位的RAW，但是后面的那段经过实验发现不是这么回事，每次生成的字符串都千差万别，不知道为什么。
在具体应用中，除了可以用来插入生成唯一的标识符外，还可以用来取表中的任意一条记录：
select * from (select * from t2 order by sys_guid()) where rownum=1;