SQLメタデータを保持しながら、複数の時間列を1つに並べ替える方法はありますか？

union 異なる時間のいくつかのクエリ間：

(SELECT id, a_time AS time, metadata1, metadata2
 FROM   mytable)
UNION
(SELECT id, b_time AS time, metadata1, metadata2
 FROM   mytable)
UNION
(SELECT id, c_time AS time, metadata1, metadata2
 FROM   mytable)
UNION
(SELECT id, d_time AS time, metadata1, metadata2
 FROM   mytable)
ORDER BY time