Which engineer id has visited the most?
select t1.engineer_id from maintenance_engineers as t1 join engineer_visits as t2 group by t1.engineer_id order by count ( * ) desc limit 1
What is that engineer's full name, please?
select t1.engineer_id, t1.first_name, t1.last_name from maintenance_engineers as t1 join engineer_visits as t2 group by t1.engineer_id order by count ( * ) desc limit 1
What are all of her visit start datetimes and visit end datetimes?
select visit_start_datetime, visit_end_datetime from engineer_visits where engineer_id = ( select t1.engineer_id from maintenance_engineers as t1 join engineer_visits as t2 group by t1.engineer_id order by count ( * ) desc limit 1 )