Hello,

The Clients table looks like this

ID NAME
1 Client1
2 Client2
3 Client3
4 Client4

The Invoices table looks like this

ID CLIENT_ID QUANTITY UNITPRICE
1 1 10 10
2 1 15 15
3 2 10 10
4 2 5 10
5 3 3 10

The Payments table:

ID CLIENT_ID AMOUNT
1 1 100
2 1 100
3 2 50
4 2 75
5 3 30


If I run the select statement below against Invoices:

select a.id, a.name,
sum(b.quantity*b.unitprice)
from clients a, invoices b
where a.id=b.client_id
group by a.id, a.name

the result is correct
ID NAME SUM(B.QUANTITY*B.UNITPRICE)
1 Client1 325
2 Client2 150
3 Client3 30

If I run the select statement below against Payments:

select a.id, a.name,
sum(c.amount)
from clients a, payments c
where a.id=c.client_id
group by a.id, a.name

the result is correct
ID NAME SUM(C.AMOUNT)
1 Client1 200
2 Client2 125
3 Client3 30

But if I want to have in a single select statement both SUM from Invoices and SUM from Payments, the select show incorrect data

select a.id, a.name,
sum(b.quantity*b.unitprice),
sum(c.amount)
from clients a, invoices b, payments c
where a.id=b.client_id and a.id=c.client_id
group by a.id, a.name

ID NAME SUM(B.QUANTITY*B.UNITPRICE) SUM(C.AMOUNT)
1 Client1 650 400
2 Client2 300 250
3 Client3 30 30


And I don't know why

Because the join clause (your where statement)
is saying
"For each row meeting this criteria, do the Select from above".
So it runs for all clients and their invoices AND all clients and their payments.

To see that, rerun your query without the group by. Just do the join and see how many rows you get back.

If what you are looking for is a table with a client and their SUM invoice amounts and SUM payment amounts you should do the SUM./ Group By FIRST and then you can join those.

--what you might be looking for
with ci as (
select 
   c.id clientid,
   c.name, 
   sum(i.quantity*i.unitprice) invoicedAmt
from 
  clients c, 
  invoices i
where c.id = i.clientid
    group by c.id,c.name
),
 cp as (
    select 
  c.id clientid,
  c.name,
  sum(p.payment) paidAmt
from 
  clients c left outer join payments p on c.id=p.clientid
group by c.id, c.name
)
select 
  c.id, 
  c.name,
  ci.invoicedAmt,
  cp.paidAmt
from 
    clients c
LEFT OUTER JOIN ci ON c.id=ci.clientid 
LEFT OUTER JOIN cp ON c.id=cp.clientid;

Gives

Result Set 6
ID NAME INVOICEDAMT PAIDAMT
1 client1 325                        305
2 client2 1300                1280
4 client4 100                         - 
3 client3 120                        110

NOTE: I shortpaid all the payments by 10$ so you can see the values from each tables.
NOTE: a,b,c are not great aliases. Try and use ones that clearly identify the source table (clients c, invoices i, payments p ...)
NOTE: used LEFT OUTER join because maybe a client has an invoice and no payment or no payments or invoices.

LiveSQL Link to see the different queries

https://livesql.oracle.com/ords/livesql/s/cpb02dav5l5wyjxbtll3sg290