Alter Table

The ALTER TABLE statement allows you to rename an existing table. It can also be used to add, modify, or drop a column from an existing table.
Renaming a table
The basic syntax for renaming a table is:
    ALTER TABLE table_name
     RENAME TO new_table_name;
For example:
    ALTER TABLE suppliers
     RENAME TO vendors;
This will rename the suppliers table to vendors.
Adding column(s) to a table
Syntax #1
To add a column to an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
     ADD column_name column-definition;
For example:
    ALTER TABLE supplier
     ADD supplier_name  varchar2(50);
This will add a column called supplier_name to the supplier table.
Syntax #2
To add multiple columns to an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
    ADD (column_1     column-definition,
          column_2     column-definition,
          …    
          column_n     column_definition );
Drop column(s) in a table
Syntax #1
To drop a column in an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
     DROP COLUMN column_name;For example:
    ALTER TABLE supplier
     DROP COLUMN supplier_name;

Modifying column(s)(datatypes) in a table
Syntax #1
To modify a column in an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
     MODIFY column_name column_type;

For example:
    ALTER TABLE supplier
     MODIFY supplier_name   varchar2(100)     not null;
This will modify the column called supplier_name to be a data type of varchar2(100) and force the column to not allow null values.
Syntax #2
To modify multiple columns in an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
    MODIFY (     column_1     column_type,
          column_2     column_type,
          …    
          column_n     column_type );
Rename column(s) in a table
(NEW in Oracle 9i Release 2)
Syntax #1
Starting in Oracle 9i Release 2, you can now rename a column.
To rename a column in an existing table, the ALTER TABLE syntax is:
    ALTER TABLE table_name
     RENAME COLUMN old_name to new_name;