What is the average number of police officers of all counties?
select avg(police_officers) from county_public_safety
Which county has largest number of police officers? Show its county_ID.
select county_id from county_public_safety order by police_officers desc limit 1
Which cities are located in this county?
select name from city where county_id = (select county_id from county_public_safety order by police_officers desc limit 1)