Writing a subquery in SQL, how to combine two queries? - sql

Question

My goal is to find the top 10 countries within the top 10 cities. I successfully used this query:
SELECT
COUNT(A.customer_id) AS number_of_customers,
D.country, C.city
FROM
customer A
INNER JOIN
address B ON A.address_id = B.address_id
INNER JOIN
city C ON B.city_id = C.city_id
INNER JOIN
country D ON C.country_ID = D.country_ID
WHERE
country IN ('India', 'China', 'United States', 'Japan', 'Mexico', 'Brazil', 'Russian Federation', 'Phillipines', 'Turkey', 'Indonesia')
GROUP BY
C.city, D.country
ORDER BY
number_of_customers DESC
LIMIT 10
But I would like to use a subquery rather than listing the countries which I found using a previous query:
SELECT
COUNT(A.customer_id) AS number_of_customers,
D.country
FROM
customer A
INNER JOIN
address B ON A.address_id = B.address_id
INNER JOIN
city C ON B.city_id = C.city_id
INNER JOIN
country D ON C.country_ID = D.country_ID
GROUP BY
D.country
ORDER BY
number_of_customers DESC
LIMIT 10
How can I combine these two queries correctly? I keep getting different errors when I try to replace the list of countries with in the second query I posted. I apologize if this is a stupid question; I am a beginner.
My attempt:
SELECT
COUNT(A.customer_id) AS number_of_customers,
D.country, C.city
FROM
customer A
INNER JOIN
address B ON A.address_id = B.address_id
INNER JOIN
city C ON B.city_id = C.city_id
INNER JOIN
country D ON C.country_ID = D.country_ID
WHERE
country IN (SELECT COUNT(A.customer_id) AS number_of_customers, D.country
FROM customer A
INNER JOIN address B ON A.address_id = B.address_id
INNER JOIN city C ON B.city_id = C.city_id
INNER JOIN country D ON C.country_ID = D.country_ID
GROUP BY D.country
ORDER BY number_of_customers DESC
LIMIT 10)
GROUP BY
C.city, D.country
ORDER BY
number_of_customers DESC
LIMIT 10
But I get an error
subquery has too many columns

Answer 1

For instance, in PostgreSQL you can use with queries, see the Documentation:
WITH top_countries AS (
SELECT count(A.customer_id) AS number_of_customers,
D.country AS country
FROM customer A
INNER JOIN address B ON A.address_id = B.address_id
INNER JOIN city C ON B.city_id = C.city_id
INNER JOIN country D ON C.country_ID = D.country_ID
GROUP BY D.country
ORDER BY number_of_customers DESC
LIMIT 10
)
SELECT count(A.customer_id) AS number_of_customers,
D.country, C.city
FROM customer A
INNER JOIN address B ON A.address_id = B.address_id
INNER JOIN city C ON B.city_id = C.city_id
INNER JOIN country D ON C.country_ID = D.country_ID
WHERE country IN (SELECT tc.country FROM top_countries)
GROUP BY C.city,D.country
ORDER BY number_of_customers DESC
LIMIT 10