Transformación de fila a columna de diferentes tablas y diferentes números de filas (unión) en MySQL versión 8.0.17 usando Pivot

Básicamente, desea una full join , que MySQL no admite. Un método para manejar esto es union all y agregación:

SELECT MAX(CASE WHEN which = 'Q400' THEN contents END) as Q400,
       MAX(CASE WHEN which = 'Q401' THEN contents END) as Q401       
c2.contents Q410
FROM ((SELECT 'Q400' as which, c.*,
              ROW_NUMBER() OVER (ORDER BY sID) as seqnum
       FROM t_contents_q400 c
      ) UNION ALL
      (SELECT 'Q401' as which, c.*,
              ROW_NUMBER() OVER (ORDER BY sID) as seqnum
       FROM t_contents_q401 c
      )
     ) q
GROUP BY seqnum;