Get top 10 cities per country

Question

Hi
I am trying to fetch top 10 cities by population for countries listed in the IN clause.
select * from (
Select city_name, population...
from test
where country in ('China','India', 'USA')
order by population
where rownum <=10.
What I need as an output is total of 30 rows in this case, 10 for India, 10 for US and 10 for China.
What am I doing wrong here? I get more rows.
Any help appreciated.
Thx!

Answer 1

You didn't specify a version.
I am assuming one with analytics available
DROP TABLE CITIES;
CREATE TABLE CITIES
(COUNTRY VARCHAR2(30)
,CITY VARCHAR2(30)
,POPULATION NUMBER
INSERT INTO CITIES (COUNTRY,CITY,POPULATION)
SELECT
'COUNTRY'||FLOOR(DBMS_RANDOM.VALUE(1,4)) COUNTRY
,'CITY'||ROWNUM CITY
,TRUNC(DBMS_RANDOM.VALUE(200,35000000) ) POPULATION
FROM
DUAL
CONNECT BY LEVEL <= 200
COMMIT;
SELECT
COUNTRY
,CITY
,POPULATION
,RANKING
FROM
SELECT
COUNTRY
,CITY
,POPULATION
,RANK() OVER (PARTITION BY COUNTRY ORDER BY COUNTRY,POPULATION) RANKING
FROM
CITIES)
WHERE
RANKING <=10
ORDER BY COUNTRY,RANKING DESC;
COUNTRY                        CITY                           POPULATION    RANKING
COUNTRY1                       CITY122                           4858897         10
COUNTRY1                       CITY133                           4819304          9
COUNTRY1                       CITY73                            4045273          8
COUNTRY1                       CITY101                           3285468          7
COUNTRY1                       CITY90                            3222442          6
COUNTRY1                       CITY142                           2996082          5
COUNTRY1                       CITY70                            2799399          4
COUNTRY1                       CITY49                            2459511          3
COUNTRY1                       CITY99                             914365          2
COUNTRY1                       CITY100                            209141          1
COUNTRY2                       CITY16                            5825305         10
COUNTRY2                       CITY195                           5152174          9
COUNTRY2                       CITY63                            5017615          8
COUNTRY2                       CITY165                           5010887          7
COUNTRY2                       CITY42                            4267026          6
COUNTRY2                       CITY50                            3255312          5
COUNTRY2                       CITY115                           3048442          4
COUNTRY2                       CITY176                           2172578          3
COUNTRY2                       CITY79                            1815271          2
COUNTRY2                       CITY4                             1039363          1
COUNTRY3                       CITY44                            3094211         10
COUNTRY3                       CITY45                            3037433          9
COUNTRY3                       CITY15                            2472461          8
COUNTRY3                       CITY36                            1710581          7
COUNTRY3                       CITY131                           1346831          6
COUNTRY3                       CITY192                           1289185          5
COUNTRY3                       CITY92                             894559          4
COUNTRY3                       CITY68                             638714          3
COUNTRY3                       CITY61                             299477          2
COUNTRY3                       CITY157                            227579          1
30 rows selected.