# The  below following query returns all the objects contained in Database  schema:
SELECT object_name, object_type FROM USER_OBJECTS; 
# Verify the all objects : 

SELECT owner, object_name, object_type FROM ALL_OBJECTS;  

# DBA object : 

SELECT owner, object_name, object_type FROM SYS.DBA_OBJECTS;

# What is the Dual Table :

It is the dummy table (Pseudo table).  It has one column, DUMMY, defined to be VARCHAR2(1). DUAL is in the schema of the user SYS but is accessible by the name DUAL to all users.


SQL> select user from dual;

USER
------------------------------
HR

SQL> select sysdate from dual;

SYSDATE
---------
22-JUN-15

SQL>  select
  2        to_char(to_date('25-MAR-1956','dd-mon-yyyy'),'day')
  3  from dual;

TO_CHAR(T
---------
sunday


Generate 1 to N no using below query:
(Change the Level <=100 if you wanted to generate 1 to 100)

CONNECT BY LEVEL : This condition would be identifies the relationship between parent rows and child rows of the hierarchy.
Syntax 1 CONNECT BY [NOCYCLE] <condition> START WITH <condition>
Syntax 2  START WITH <condition> CONNECT BY [NOCYCLE] <condition> 



SQL> SELECT LEVEL n FROM DUAL CONNECT BY LEVEL <= 10;

         N
----------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10

10 rows selected.

SQL>