LAG & LEAD Analytic function in Oracle

The LAG and LEAD  and  other analytic functions are available in Oracle to  give access to multiple rows within a table, without the need for a self-join. These function will be very handy.
For example: 
To get previous record column value
To calculate difference between column value. 



http://oracle-base.com/articles/misc/lag-lead-analytic-functions.php

http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions070.htm

http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions074.htm#i83834

http://oracle-base.com/articles/misc/rank-dense-rank-first-last-analytic-functions.php