Get first date from timestamp in SQL

I have in my Moodle `db` `table` for every `session` `sessid` and `timestart`. The table looks like this: +----+--------+------------+ | id | sessid | timestart | +----+--------+------------+ | 1 | 3 | 1456819200 | | 2 | 3 | 1465887600 | | 3 | 3 | 1459839600 | | 4 | 2 | 1457940600 | | 5 | 2 | 1460529000 | +----+--------+------------+ How to get for `every` `session` the `first` `date` from the `timestamps` in `SQL`?