Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE continent (cid, continent) AS
  SELECT 1, 'Africa'        FROM DUAL UNION ALL
  SELECT 2, 'Asia'          FROM DUAL UNION ALL
  SELECT 3, 'South America' FROM DUAL;
3 rows affected

CREATE TABLE countries (id, country_name, is_preferred, cities) AS
  SELECT 1, 'Dominican Republic', 'Y', 'Kinshasa'  FROM DUAL UNION ALL
  SELECT 1, 'Nigeria',            'Y', 'Lagos'     FROM DUAL UNION ALL
  SELECT 1, 'Egypt',              'Y', 'Cairo'     FROM DUAL UNION ALL
  SELECT 2, 'China',              'N', 'Shanghai'  FROM DUAL UNION ALL
  SELECT 2, 'Pakistan',           'Y', 'Karachi'   FROM DUAL UNION ALL
  SELECT 3, 'Brazil',             'Y', 'Sao Paulo' FROM DUAL UNION ALL
  SELECT 3, 'Peru',               'Y', 'Lima'      FROM DUAL;
7 rows affected

SELECT c.continent,
       LISTAGG(ct.cities, ',') WITHIN GROUP (ORDER BY cities) AS cities
FROM   continent c
       INNER JOIN countries ct
       ON c.cid = ct.id
WHERE  ct.is_preferred = 'Y'
GROUP BY
       c.continent
HAVING COUNT(1) > 1
ORDER BY
       c.continent;


CONTINENT
CITIES


Africa
Cairo,Kinshasa,Lagos

South America
Lima,Sao Paulo