Question

I have these SQL tables

Supplier

 SUPPLIERID | SUPPLIERNAME
 1          | sup1
 2          | sup2
 3          | sup3

Sold

ITEMID | SOLD | SUPPLIER ID | DATE
pen    | 10   | 1           | 2011-10-21
pen    | 5    | 1           | 2011-10-22
pen    | 5    | 1           | 2011-10-23
pen    | 20   | 3           | 2011-10-24

Bought

 ITEMID | BOUGHT | SUPPLIER ID | DATE
 pen    | 20     | 1           | 2011-9-21
 pen    | 5      | 3           | 2011-9-24

I’m currently using this script but it doesn’t seem to work as the SUM() of sold and bought is tripled. I suspect the reason is because there are 3 rows in sold but only 2 in bought. Any idea how to fix this?

   SELECT suppliername, SUM(sold) as sold, SUM(bought) AS bought
     FROM sold s
LEFT JOIN supplier sp
       ON s.supplierid = sp.supplierid
LEFT JOIN bought b
       ON sp.supplierid = b.supplierid
 GROUP BY suppliername

Answer 1

SELECT suppliername, Sold, Bought
FROM Supplier sp
LEFT JOIN (SELECT SupplierId, SUM(Sold) Sold
           FROM sold
           GROUP BY SupplierId) s
ON sp.SupplierId = s.SupplierId
LEFT JOIN (SELECT SupplierId, SUM(bought) bought
           FROM bought
           GROUP BY SupplierId) b
ON sp.SupplierId = b.SupplierId

Basically, what I’m doing is calculating the amount sold and the amount bought for every supplierId separately (in the LEFT JOINs), so I always have the total amount sold and total amount bought. The querys that I’m using in the LEFT JOINs are called derived tables, since they are acting like a table on the general query. Hope this helps you get a more clear idea of what I did.