我想获得多列时间戳的最小值。这是我的数据
Id timestamp 1 timestamp 2 timestamp 3
136 2014-08-27 17:29:23 2014-11-05 13:02:18 2014-09-29 22:26:34
245 2015-09-06 15:46:00 NaN NaN
257 2014-09-29 22:26:34 2016-02-02 17:59:54 NaN
258 NaN NaN NaN
480 2016-02-02 17:59:54 2014-11-05 13:02:18 NaN
我想获得最小的时间戳
Id minimal
136 2014-08-27 17:29:23
245 2015-09-06 15:46:00
257 2014-09-29 22:26:34
258 NaN
480 2014-11-05 13:02:18