How to build "dummy columns" for UNION?

Hi,
You can use,

 SELECT "City", "Country", "Continent" from table1  
UNION  
SELECT "City", "Country", ' ' as "Continent"  from table2

or

 SELECT "City", "Country", "Continent" from table1  
    UNION  
    SELECT 

"City", "Country", NULL as "Continent"  from table2

It considers "Continent" as null in table2

You can always add "virtual" columns:

SELECT "City", "Country", "Continent" from table1  
UNION  
SELECT "City", "Country", NULL AS "Continent" from table2