Aggregate functions return wrong values when joining more tables

Answer 1

Simple solution: Use two queries.

Otherwise you can do your aggregated calculation in a subquery (on the whole table, not per row) then JOIN the result of the subquery with the addresses table to get your extra data. Try this:

SELECT *
FROM customer T1
LEFT JOIN
(
    SELECT custId,
           SUM(total) AS sum_total,
           COUNT(total) AS count_total
    FROM orders
    -- WHERE ...
    GROUP BY custId
) T2
ON T1.custId = T2.custId
-- WHERE ...

Answer 2

Try something like:

select c.custId, max(c.custName) custName, ...
       a.addrId, max(a.addrLine1) addrLine1, ...
       sum(o.total) order_total, count(o.total) order_count
from customer c 
left join orders o on c.custId = o.custId
left join cust_addresses a on c.custId = a.custId
group by c.custId, a.addrId;

assuming you want all customers, whether or not they have any orders or addresses.