SQL从一列中的两列获取记录

Question

I am trying to get records from two columns in such a way that the output contains 1 record from 1st col then 1st record from 2nd col.

the senario is like this

ColA     ColB
1         a
2         b
3         c

Output
1
a
2
b
3
c

Answer 1

SELECT x.Output FROM
(
   SELECT Output = CAST(ColA AS VARCHAR(10)), 
          RN = ROW_NUMBER()OVER(ORDER BY ColA,ColB)
   FROM TableName 

   UNION ALL 

   SELECT Output = ColB, 
          RN = ROW_NUMBER()OVER(ORDER BY ColA,ColB)
   FROM TableName 
) X
ORDER BY x.RN

Demo: http://sqlfiddle.com/#!6/7f3cf/4/0

Answer 2

Here is an approach without window functions, assuming that ColA specifies the ordering for the rows:

select output
from ((select cola as output, cola as ordering, 'a' as which
       from table t
      ) union all
      (select colb, cola, 'b'
       from table t
      )
     ) t
order by ordering, which;

Note that this will guarantee that the values are interleaved in the result set.