I want to combine two columns in such a way that the second column gets added below the first column.
Col 1: A B C
Col 2: D E F
Col : A B C D E F
UNION ALL to get the two columns as one single column:
select col1 from tablename UNION ALL select col2 from tablename
If you absolutely want col1 values before col2 values, wrap it up in a derived table and add an
select col from ( select col1 as col, 1 as ob from tablename UNION ALL select col2, 2 as ob from tablename ) dt order by ob