Help with join

1

@ivan

What db matters here as some do not support outer joins well
But you need an outer join on City to make sure you get one row for every city

2

like this

with common_cte as  (
   select sales.cityFK , sales.employeeFK, sales.sales
   from sales
   where sales.employeeFK = 10
  )
  select city.id, city.cityname, case when common_cte.sales is null then 0 else common_cte.sales end
    from city
     left outer join common_cte on city.id = common_cte.cityFK

which gives you a row for every city regardless of what employee id you put in (something the other answer doesnt do)