If you have an existing column in a table and you need that column any more, you can delete it with ALTER TABLE ... DROP COLUMN statement. Here is an example SQL script:

SQL> CREATE TABLE emp_dept_90

2 AS SELECT * FROM employees WHERE department_id=90;

Table created.

SQL> SELECT last_name FROM emp_dept_90;

LAST_NAME

-------------------------

King

Kochhar

De Haan

SQL> ALTER TABLE emp_dept_90 DROP COLUMN last_name;

Table altered.

SQL> SELECT last_name FROM emp_dept_90;

ERROR at line 1:

ORA-00904: "LAST_NAME": invalid identifier

As you can see the column "last_name" is gone.