MySQL sums from multiple tables - mysql

Question

I have multiple tables and I need to aggregate the data from all of them, but it seems that I always get the wrong results for the sums. What am I doing wrong?
customers
ID Name
1 c1
2 c2
3 c3
budget
ID Cust_ID Value
1 1 100
2 1 300
3 2 600
4 3 450
forecast
ID Cust_ID Value
1 1 200
2 1 500
3 2 100
4 2 700
5 3 550
orders
ID Cust_ID Net_Sales
1 1 100
2 1 200
3 1 300
4 2 400
5 3 500
Here is the expected result:
ID Name sum(budget.Value) sum(forecast.Value) sum(orders.Net_Sales) count(orders.ID)
1 c1 400 700 600 3
2 c2 600 800 400 1
3 c3 450 550 500 1
And here's what I've tried so far:
SELECT customers.ID, customers.Name, sum(budget.Value), sum(forecast.Value), sum(orders.Net_Sales), count(orders.ID)
FROM customers
INNER JOIN budget ON budget.Cust_ID = customers.ID
INNER JOIN forecast ON forecast.Cust_ID = customers.ID
INNER JOIN orders ON orders.Cust_ID = customers.ID
GROUP BY customers.ID
ORDER BY customers.ID ASC

Answer 1

You are joining along multiple dimensions, which multiplies the results.
A simple solution is correlated subqueries:
SELECT c.ID, c.Name,
(SELECT SUM(b.Value)
FROM budget b
WHERE b.Cust_ID = c.ID
) as budget,
(SELECT SUM(f.Value)
FROM forecast f
WHERE f.Cust_ID = c.ID
) as forecast,
(SELECT SUM(o.Net_Sales)
FROM orders o
WHERE o.Cust_ID = c.ID
) as net_sales
FROM customers c
ORDER BY c.ID ASC;
With the right indexes in the second tables (budget(cust_id, value), and so on), this may actually be faster than a JOIN approach.