A union generates more rows not more columns. Does this return what you expect?

SELECT col1, col2, col3 FROM Table1 WHERE (EXISTS (SELECT col5 FROM Table2 WHERE col1 =c5))

UNION ALL

SELECT col4, col5, col6 FROM Table2

There will only be 3 columns returned from this statement. Their headings would be col1, col2, col3. The data for col4, col5, col6 will be there but the first heaing will be used.