Why group by requires all non aggregate columns

Question

In sql, this query has invalid syntax:

select country,city, sum(income) from table1 group by country.

Because the group by expects country and city, not just country. But why is this the case? Because it is possible that I want the result to be grouped by country only. So in below table:

country   city            income

canada    toronto          100
canada    montreal          100
us        LA               200
us        NY               300

If I want to output:

country   city           sum( income)

canada    toronto          200
canada    montreal          200
us        LA               500
us        NY               500

What should be my group by clause?

Answer 1

if you are using Sql Server you can do this

select 
  country,
  city, 
  sum(income) over (partition by country)
from 
  table1 

Or if you are using another database you can use a sub query

  select 
      t1.country,
      t1.city, 
      (select sum(t2.income) from table1 t2 where t1.country = t2.country)
    from 
      table1 t1