Oracle随机函数

 dbms_random 

 

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

 

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