Проблема заключается в g
в select * from g
, это должно быть
SELECT * FROM (
SELECT COUNT(c.city) AS number_cities, co.country
FROM city c
JOIN country co ON c.country_id= co.country_id
GROUP BY co.country ORDER BY country) g
WHERE number_cities>= 20
, или вы можете удалить подзапрос и использовать having
вместо
SELECT COUNT(c.city) AS number_cities, co.country
FROM city c
JOIN country co ON c.country_id= co.country_id
GROUP BY co.country
HAVING COUNT(c.city)>=20
ORDER BY country