select case when aa.time>bb.time then aa.id else bb.id end id,
case when aa.time>bb.time then aa.send else bb.send end send,
case when aa.time>bb.time then aa.recvid else bb.recvid end recvid,
case when aa.time>bb.time then aa.time else bb.time end time
from mytab aa
inner join mytab bb on aa.send=bb.recvid and aa.recvid=bb.send