Yes this is a valid SQL statement use similar ones a great deal. The problem with nested queries is they can only return 1 record per match. Another problem can be performance it has to run for every record in order tables and it occurs pre filter conditionsA nested query Select orders.*, (SELECT MAX(ol_timestamp) FROM orders_log where orders_log.o_id = orders.oid)>From orders That won't give the desired results. I don't think the SQL parser will even accept it.
Still another option is using a join Select orders.*, ol_timestamp From orders left join (SELECT MAX(ol_timestamp), o_id FROM orders_log group by o_id) as JoinQuery on JoinQuery.o_id = orders.o_idThat won't give the desired results either. If you change "left" to "inner" you will be closer though. Both of your queries will retrieve the entire orders table with a timestamp of some sort from the orders_log table.
"How can I select all from orders and the last (latest) entry from the orders_log?"