Google search console data - find the newly appeared keyword for the month
I figured it out after a brief study, here it is,

SELECT * FROM sconsole WHERE sconsole.query IN
                (SELECT  tbl.query FROM
                  (  SELECT  s1.* FROM sconsole s1 WHERE s1.month = '{$params['m1']}' UNION ALL SELECT  s2.* FROM sconsole s2 WHERE s2.month = '{$params['m2']}'
                  ) AS tbl  GROUP BY  tbl.query HAVING COUNT(tbl.query) = 1)


works like a charm.