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How to generate a sequence of numbers within Oracle,   

Example of Sequences in SQL: 

Assume we have executed the following create table statement. 

create table employees (
  eno     number(4) primary key, 
  ename   varchar(30),
  zip     number(5), 
  hdate   date);   

Now we create a sequence object.  We can create numerous sequence objects with 
different names, starting values and increments.  The default increment is 1.  

As an example, we could 

  create sequence EnumberSeq start with 1000; 


Now execute the following insert statements: 

  insert into employees values 
   (EnumberSeq.nextval, 'Tom Jones', 67226,'12-DEC-00'); 

Jones will be assigned a value of 1000 for column eno. 


  insert into employees values 
   (EnumberSeq.nextval, 'Ruth Brown',67226,'12-DEC-00'); 

Brown will be assigned a value of 1001 for column eno. 
 

If we want to insert the current value of the sequence EnumberSeq into 
a column of a row we can do that by using the reference:  

    EnumberSeq.currval  

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Example of Dates in SQL: 

Default format is 'DD-MON-YY"   
  where DD is 2 digit day of month 
        MON is 3 letter abbreviation for month 
        YY is last 2 digits of year 

Other formats include 
        MM   2 digit month 
        DY   3 letter abbreviation for day  
      YYYY   4 digit year  

Finding the current date: 

   Select sysdate from dual;    dual is a system defined dummy table with one column and one row.

The to_char function allows us to reformat a date  
  
    Select to_char(sysdate, 'DD-MON')
    from dual; 

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Some useful tables that are part of the Oracle database catalog: 

   user_catalog      contains info about table name and table type 

   user_tab_columns  contains information about the columns in the tables  

   user_tables       contains much detailed information about the tables  

   user_views        contains information about views  

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