SQL> CREATE OR REPLACE PACKAGE id_generator
  2  AUTHID CURRENT_USER
  3  AS
  4      FUNCTION convert_0_to_59_to_char (
  5          p_num   IN  NATURAL
  6      )
  7      RETURN VARCHAR2;
  8      
  9      FUNCTION make_id (
 10          p_length    IN  POSITIVE    DEFAULT 9
 11      )
 12      RETURN VARCHAR2;
 13  END id_generator;
 14  /
  
Package created.
  
SQL> CREATE OR REPLACE PACKAGE BODY id_generator AS
  2      FUNCTION convert_0_to_59_to_char (
  3          p_num   IN  NATURAL
  4      )
  5      RETURN VARCHAR2
  6      IS
  7          l_char      VARCHAR2(1);
  8      BEGIN
  9          IF p_num BETWEEN 0 AND 9 THEN
 10              l_char := CHR(48 + p_num);
 11          ELSIF p_num BETWEEN 10 AND 35 THEN
 12              l_char := CHR(55 + p_num);
 13          ELSIF p_num BETWEEN 36 AND 46 THEN
 14              l_char := CHR(61 + p_num);
 15          ELSIF p_num BETWEEN 47 AND 48 THEN
 16              l_char := CHR(62 + p_num);
 17          ELSIF p_num BETWEEN 49 AND 59 THEN
 18              l_char := CHR(63 + p_num);
 19          ELSE
 20              l_char := NULL;
 21          END IF;
 22          RETURN (l_char);
 23      END convert_0_to_59_to_char;
 24      
 25      FUNCTION make_id (
 26          p_length    IN  POSITIVE    DEFAULT 9
 27      )
 28      RETURN VARCHAR2
 29      IS
 30          l_num       NATURAL;
 31          l_id        VARCHAR2(100);
 32      BEGIN
 33          FOR i IN 1..LEAST(p_length,100) LOOP
 34              l_num := MOD(ABS(DBMS_RANDOM.RANDOM),60);
 35              l_id := l_id
 36                      || convert_0_to_59_to_char(l_num);
 37          END LOOP;
 38          RETURN (l_id);
 39      END make_id;
 40  END id_generator;
 41  /
  
Package body created.
  
SQL> SELECT id_generator.make_id FROM DUAL;
  
MAKE_ID
---------------------------------------------------------------------------------------------------------
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SQL> SELECT id_generator.make_id FROM DUAL;
  
MAKE_ID
---------------------------------------------------------------------------------------------------------
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SQL> SELECT id_generator.make_id FROM DUAL;
  
MAKE_ID
---------------------------------------------------------------------------------------------------------
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SQL> SELECT id_generator.make_id(20) FROM DUAL;
  
ID_GENERATOR.MAKE_ID(20)
---------------------------------------------------------------------------------------------------------
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SQL> SELECT id_generator.make_id(90) FROM DUAL;
  
ID_GENERATOR.MAKE_ID(90)
---------------------------------------------------------------------------------------------------------
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SQL>