How to use dummy column?

select 

 department_id,TO_NUMBER(null) location, hire_date

from employees

union

select 

 department_id, location_id, TO_DATE(null) hired 

from departments

In this example, the name location is given as the dummy column heading. The TO_NUMBER function is used in the first query to match the NUMBER data type of the LOCATION_ID column retrieved by the second query. Similary, the TO_DATE function in the second query is used to match the DATE data type of the HIRE_DATE column retrieved by the first query.

select

 employee_id,job_id,salary 

from

 employees

union

select 

 employee_id,job_id,0

from

 job_history;

Or, you can use this way.