mysql sum() two joined tables, multiplies result

Question

i have two tables; invoices & invoiceitems.
invoiceitems contains the items on each invoice

eg:

invoices
----------------------------------
| id  |status| net | tax | total |
----------------------------------
| 72  |paid  | 100 | 120 |  220  |
| 73  |unpaid| 50  | 5   |  55   |
| 74  |paid  | 400 | 45  |  445  |
| 75  |paid  | 250 | 67  |  317  |


invoiceitems
-------------------------------
| invoiceid |itemdescription |
-------------------------------
| 72        | apples         |
| 72        | pears          |
| 72        | oranges        |
| 73        | lemons         |
| 73        | oranges        |

as you can see, in the example invoice number 72 has 3 items

i want to search my invoices for certain things, and display a count of certain fields.

but my problem is that the sum value seems to get multiplied by the number of fields there are in the second table.

$sql = "SELECT COUNT(DISTINCT invoices.id) AS num, 
SUM(CASE invoices.status WHEN 'Paid' THEN 1 ELSE 0 END) AS numpaid, 
SUM(CASE invoices.status WHEN 'Paid' THEN invoices.total ELSE 0 END) AS sumtotal,
FROM invoices 
LEFT JOIN invoiceitems ON invoices.id=invoiceitems.invoiceid
WHERE invoices.id LIKE :invoiceid 
AND IFNULL(opcinvoiceitems.itemdescription, '')  LIKE :itemdescription
AND invoices.net LIKE :net 
AND invoices.tax LIKE :tax 
AND invoices.total LIKE :total
AND ......" 

so using the above, the total for invoice 72 would be multiplied by 3

i'm really sorry, i know this is really badly explained but i cant explain it any other way, been searching for ages but cant find a solution. hope someone can help. thanks

Answer 1

When you do a join, you produce records created by matching up ones from the original tables. Thus, you will have 3 records for invoice #72, each created by matching up the single invoices record for #72 with each of the invoice items for #72. Each combined record will have the same total (in this case, 220), and thus the sum would be 3 times that.

It sounds like you just want total, then; you could just use total directly, or you could take your sum and divide it by the count (which you appear to also be computing).

Answer 2

One way to do what you want is to pre-aggregate the invoiceItems table before joining:

SELECT COUNT(i.id) AS num, 
       SUM(CASE i.status WHEN 'Paid' THEN 1 ELSE 0 END) AS numpaid, 
       SUM(CASE i.status WHEN 'Paid' THEN i.total ELSE 0 END) AS sumtotal,
FROM invoices  i LEFT JOIN
     (select ii.invoiceid, sum(. . .) as . . .
      from invoiceitems ii
      where IFNULL(ii.itemdescription, '')  LIKE :itemdescription AND
      group by ii.invoiceid
     ) ii
     ON i.id = ii.invoiceid
WHERE i.id LIKE :invoiceid AND
      i.net LIKE :net  AND
      i.tax LIKE :tax AND
      i.total LIKE :total AND .....

Your query doesn't actually use invoiceitems in the from clause, so it is hard to provide a more detailed example.