各日付の最大日時を見つけるサブクエリに参加できます。
SELECT t1.trade_time, t1.price
FROM yourTable t1
INNER JOIN
(
SELECT DATE(trade_time) AS trade_date, MAX(trade_time) AS max_trade_time
FROM yourTable
GROUP BY DATE(trade_time)
) t2
ON t2.trade_date = DATE(t1.trade_time) AND
t2.max_trade_time = t1.trade_time
ORDER BY
t1.trade_time;