Is it possible to select subqueries - sql

Question

Consider the query below:
SELECT DISTINCT ON (ser.id) *
FROM server ser
LEFT JOIN subscription sub ON ser.id = sub.server_id
WHERE (
COUNT(SELECT err.id FROM error err WHERE ser.id = err.id) > 0
OR SUM(SELECT pay.amount FROM payment pay WHERE ser.id = pay.id) > 0
);
Here, a list of unique servers that are being subscribed to and that has errors or payments is returned.
However, instead of returning all server columns (*), I want to return the server id, the number of errors and the sum of payments. For example, the initial selection should look like this:
SELECT DISTINCT ON (ser.id) ser.id, countErrors, sumPayments
Selecting ser.id is straight forward, but how can countErrors and sumPayments be selected from the aggregate functions "count" and "sum" (considering that they are conditions in a WHERE clause)?
I imagined the "where" conditions would look something like this:
COUNT(SELECT err.id FROM error err WHERE ser.id = err.id) AS countErrors > 0
OR SUM(SELECT pay.amount FROM payment pay WHERE ser.id = pay.id) AS sumPayments > 0
Is it possible to do this? If so, how can it be achieved?
Test data is shown below:
server
+----+
| id |
+----+
| 1 |
+----+
| 2 |
+----+
| 3 |
+----+
| 4 |
+----+
subscription
+----+-----------+
| id | server_id |
+----+-----------+
| 1 | 1 |
+----+-----------+
| 2 | 2 |
+----+-----------+
| 3 | 2 |
+----+-----------+
| 4 | 3 |
+----+-----------+
| 5 | 3 |
+----+-----------+
error
+----+-----------+
| id | server_id |
+----+-----------+
| 1 | 1 |
+----+-----------+
| 3 | 4 |
+----+-----------+
payment
+----+-----------+--------+
| id | server_id | amount |
+----+-----------+--------+
| 1 | 1 | 200 |
+----+-----------+--------+
| 2 | 2 | 200 |
+----+-----------+--------+
| 3 | 2 | 100 |
+----+-----------+--------+
Wanted result from test data:
+-----------+-------------+-------------+
| server_id | countErrors | sumPayments |
+-----------+-------------+-------------+
| 1 | 1 | 200 |
+-----------+-------------+-------------+
| 2 | 0 | 300 |
+-----------+-------------+-------------+
Server#4 has no subscription, so it should be left out.
Server#3 has a subscription, but no errors or payments, so should be left out.
Server#1 and server#2 both have subscription and payments and/or errors.

Answer 1

Unless I'm missing something, I would just write your query as follows. Perform the aggregation of errors and payments in separate bona fide subqueries, and join to them. Also, there is a join to the subscription table, but this only exists to filter off servers having no subscription. Finally, the WHERE clause removes any servers which do not either have some errors or payments.
SELECT
s.id AS server_id,
COALESCE(e.countErrors, 0) AS countErrors,
COALESCE(p.sumPayments, 0) AS sumPayments
FROM server s
INNER JOIN
(
SELECT DISTINCT server_id
FROM subscription
) su
ON s.id = su.server_id
LEFT JOIN
(
SELECT server_id, COUNT(*) AS countErrors
FROM error
GROUP BY server_id
) e
ON s.id = e.server_id
LEFT JOIN
(
SELECT server_id, SUM(amount) AS sumPayments
FROM payment
GROUP BY server_id
) p
ON s.id = p.server_id
WHERE
p.sumPayments > 0 OR
e.countErrors > 0
ORDER BY
s.id;
Demo

Answer 2

The mistake here is to put the COUNT outside of the SELECT, it needs to go inside:
(SELECT COUNT(err.id) FROM error err WHERE ser.id = err.id) > 0
OR (SELECT SUM(pay.amount) FROM payment pay WHERE ser.id = pay.id) > 0