Two ways
1. DELETE
2. TRANCATE


BY using delete----
U can delete one nd all rows by using delete command
syntax-
        delete from tablename where column_name=column_value;  
                (it will delete the row related to given column value)
        delete * from tablename;
                 (it will remove all the rows from the table)
NOTE-1- In case of delete stil high water mark remain set to same position even   
               after deleteing all the rows.
          2- Also the rows never release the memory space .
          3- U can rollback in case of delete.



BY using trancate-----
U can delete all the rows by using the trancate command.
syntax-
        trancate table tablename;

NOTE- 1- in case of trancate high water mark get set to zero.
           2- all the rows get deleted nd relase the memory space but structure,
                index nd constraint remain the same.
           3- U can never rollback in case of trancate command.