MySQL SELECT. LIMIT number of values from one column - mysql

Question

Task: From MySQL standard database "world" select all the cities with population more then a million, limiting number of countries to 10.
If I do this:
SELECT country.Name, city.Name, city.Population
FROM country
INNER JOIN city
ON country.Code = city.CountryCode
WHERE city.Population >= 1000000
LIMIT 10;
it produces this:
+-------------+--------------+------------+
| Name | Name | Population |
+-------------+--------------+------------+
| Afghanistan | Kabul | 1780000 |
| Algeria | Alger | 2168000 |
| Angola | Luanda | 2022000 |
| Argentina | Buenos Aires | 2982146 |
| Argentina | La Matanza | 1266461 |
| Argentina | Córdoba | 1157507 |
| Armenia | Yerevan | 1248700 |
| Australia | Sydney | 3276207 |
| Australia | Melbourne | 2865329 |
| Australia | Brisbane | 1291117 |
+-------------+--------------+------------+
Argentina, for example, is repeated 3 times. I need to limit number of countries to 10, not number of rows.
GROUP BY won't do since I need all the cities, I don't need them grouped.
The expected result would be this:
+--------------------+----------------------------+------------+
| Name | Name | Population |
+--------------------+----------------------------+------------+
| Afghanistan | Kabul | 1780000 |
| Algeria | Alger | 2168000 |
| Angola | Luanda | 2022000 |
| Argentina | Buenos Aires | 2982146 |
| Argentina | La Matanza | 1266461 |
| Argentina | Córdoba | 1157507 |
| Armenia | Yerevan | 1248700 |
| Australia | Sydney | 3276207 |
| Australia | Melbourne | 2865329 |
| Australia | Brisbane | 1291117 |
| Australia | Perth | 1096829 |
| Azerbaijan | Baku | 1787800 |
| Bangladesh | Dhaka | 3612850 |
| Bangladesh | Chittagong | 1392860 |
| Brazil | São Paulo | 9968485 |
| Brazil | Rio de Janeiro | 5598953 |
| Brazil | Salvador | 2302832 |
| Brazil | Belo Horizonte | 2139125 |
| Brazil | Fortaleza | 2097757 |
| Brazil | Brasília | 1969868 |
| Brazil | Curitiba | 1584232 |
| Brazil | Recife | 1378087 |
| Brazil | Porto Alegre | 1314032 |
| Brazil | Manaus | 1255049 |
| Brazil | Belém | 1186926 |
| Brazil | Guarulhos | 1095874 |
| Brazil | Goiânia | 1056330 |
| United Kingdom | London | 7285000 |
| United Kingdom | Birmingham | 1013000 |
+--------------------+----------------------------+------------+
As you can see, number of countries is 10, no matter how many cities.
Of course, my database is different. I used the "world" for simplicity, generality, and availability.
In the real db I also do some filtering on "country".

Answer 1

I Hope this will work
SELECT country.Name, city.Name, city.Population
FROM country
LEFT JOIN city
ON country.Code = city.CountryCode
WHERE city.Population >= 1000000
AND city.id IN (SELECT id FROM city WHERE city.Population >= 1000000 AND country.Code = city.CountryCode LIMIT 10);

Answer 2

//So sorry try this query
SELECT country.Name, city.Name, city.Population
FROM country
LEFT JOIN city
ON country.Code = city.CountryCode
WHERE city.Population >= 1000000 and country.Code IN(SELECT distinct CountryCode FROM city WHERE Population >= 1000000 LIMIT 10);

Answer 3

Generate temporary row level id's for each of the unique country in a sub query and select rows from it applying a where condition like where temp_row_num is <= 10.
Example:
select
/*uq_country_row_num, */
country_name, city_name, population
from (
select country_name, city_name, population
, case when #pn=(#cn:=country_name)
then #r:=(#r + 1)
else #r:=1
end as uq_country_row_num
, #pn:=#cn as temp_cn
from population,
(select #pn:='', #cn:='', #r:=0) country_rown_nums
-- use order by country_name if the result set is un-ordered
order by
country_name
) all_results
where
uq_country_row_num <= 10
;
You can un comment the variable /*uq_country_row_num, */, if you want them as part of the output.
Example # SQL Fiddle
Edit:
From the comments:
You mean 10 rows per country or all records from max 10 countries?
Exactly, "all records from max 10 countries".
select
country_id,
country_name, city_name, population
from (
select
country_name, city_name, population
, case when #pn=(#cn:=country_name)
then #country_id:=#country_id
else #country_id:=(#country_id + 1)
end as country_id
, #pn:=#cn as temp_cn
from population,
(select #pn:='', #cn:='', #row_num:=0, #country_id:=0) country_row_nums
order by country_name
) all_results
where
country_id <= 10
;
Example 2 # SQL Fiddle

Answer 4

Try this once
SELECT city.Name, city.Population, newcountry.name
FROM city
INNER JOIN
(select distinct Name,Code
from country limit 10) newcountry
ON newcountry.Code = city.CountryCode
WHERE city.Population >= 1000000;

Answer 5

None of the answers worked. So I had to use the following request:
SELECT co.Code, co.Name, ci.Name, ci.Population
FROM city ci
INNER JOIN (
SELECT DISTINCT co.Code, co.Name
FROM country co
INNER JOIN city ci
ON ci.countryCode = co.Code
WHERE ci.Population > 1000000
ORDER BY co.Name
LIMIT 10
) co
ON ci.countryCode = co.Code
WHERE ci.Population > 1000000;
It seems rather clumsy and not efficient to me (notice the repetition of the condition), but for lack of a better way, this is the only solution.
Thanks to everyone for your answers anyway.