SELECT values where their foreign keys are duplicate - sql

Question

Let's say that we have clients and providers. A client can have multiple providers (like the internet, phone, TV etc) and I would like to find clients' names who have multiple providers.
create table clients
(
client_id char(8) not null,
client_name varchar(80) not null,
contract char(1) not null,
primary key (client_id)
)
create table client_provider
(
provider_id char(11) not null,
client_id char(8) not null,
primary key (provider_id, client_id),
foreign key (provder_id) references providers ON DELETE CASCADE,
foreign key (client_id) references clients ON DELETE CASCADE
);
Therefore, even without knowing anything about providers, we can know clients with multiple providers by the following relational algebra (just started learning, please correct me if I am wrong):
π client_name (
[
σ client_provider2.provider_id ≠ client_provider.provider_id ∧ client_provider2.client_id = client_provider.client_id (ρ client_provider2 (client_provider) ⨯ client_provider))
⨝ clients]
what I have tried so far (returning "not a GROUP BY expression" in line 1):
SQL> select c.client_name
2 from clients c
3 inner join client_provider cp on c.client_id = cp.client_id
4 group by cp.client_id
5 having count(*) > 1;

Answer 1

When using GROUP BY all columns used should either be in GROUP BY or in an aggregate function. To resolve the issue do the following:
Add cp.client_id in SELECT clause
Add c.client_name in GROUP BY clause
SELECT
cp.client_id,
c.client_name
FROM clients c
INNER JOIN client_provider cp
ON c.client_id = cp.client_id
GROUP BY
cp.client_id,
c.client_name
HAVING
COUNT(1) > 1

Answer 2

All non-aggregated columns must be in group by clause, now you know that.
As you commented that you want to display only client_name but not client_id (while it has to be in the group by clause), use current query as source for the final result:
select client_name
from (-- current query begins here
select cp.client_id,
c.client_name
from clients c join client_provider cp on c.client_id = cp.client_id
group by cp.client_id,
c.client_name
having count(*) > 1
-- current query ends here
);
Alternatively, you could do it by using (slightly modified) current query as a subquery:
select cl.client_name
from client cl
where cl.client_id in (select cp.client_id
from client_provider cp
group by cp.client_id
having count(*) > 1
);