Modify tables, add columns, delete columns, and modify columns in oracle

Modify tables, add columns, delete columns, and modify columns in oracle
1. Add Columns
Alter table table_name ADD (column datatype [default expr] [, column datatype...]);
For example:
SQL> ALTER TABLE emp01 ADD eno NUMBER (4 );
 
2. Modify column Definitions
For example:
SQL> ALTER TABLE emp01 MODIFY job VARCHAR2 (15)
2 DEFAULT 'cler'
 
3. Delete Columns
For example:
SQL> ALTER TABLE emp01 DROP COLUMN dno;
 
4. Modify the column name
For example:
SQL> ALTER TABLE emp01 RENAME COLUMN eno TO empno;
 
5. Modify the table name
For example:
SQL> RENAME emp01 TO employee;
 
6. Add comments
For example:
SQL> COMMENT ON TABLE employee IS 'employee information ';
SQL> COMMENT ON TABLE employee. name IS 'employee name ';

7. Evaluate the length of the field ()
For example:
Select t. * from tp_area t where substr (t. area_id,) = '03' and length (t. area_id)> 2