How To Add a New Column to an Existing Table?

If you have an existing table with existing data rows, and want to add a new column to that table,
you can use the ALTER TABLE ... ADD statement to do this. Here is an example script:
SQL> connect HR/fyicenter
Connected.
SQL> CREATE TABLE emp_dept_110
2 AS SELECT * FROM employees WHERE department_id=110;
Table created.
SQL> ALTER TABLE emp_dept_110 ADD (vacation NUMBER);
Table altered.
SQL> SELECT first_name, last_name, vacation FROM emp_dept_110;
FIRST_NAME LAST_NAME VACATION
-------------------- ------------------------- ----------
Shelley Higgins
William Gietz
This SQL script added a new column called "vacation" to the "emp_dept_110" table. NULL values
were added to this column on all existing data rows.