カスタム配列の位置に従ってポストを注文する

次のようなクエリを使用できます：

SELECT PostID, title, position
FROM (
   SELECT PostID, title, position,
          @grp := IF(@pos = position, @grp + 1,
                     IF(@pos := position, 1, 1)) AS grp
   FROM mytable
   CROSS JOIN (SELECT @grp := 0, @pos := '') AS vars
   ORDER BY position, title) AS t
ORDER BY grp, FIELD(position, 'Middle', 'Bottom', 'Top')   

デモはこちら