MySQL GROUP BY multiple columns from different tables

Question

I\'ve got the following table layouts:

Table Data
+----------+-------------------------+
| Field    | Type                    |
+----------+-----------------         


        

Answer 1

Your query

SELECT sum(d.data) as total
FROM data d, ta, tb
WHERE
(d.type LIKE "type_a" AND d.type_id = ta.id) 
OR 
(d.type LIKE "type_b" AND d.type_id = tb.id) 
GROUP BY a.customer_id, b.customer_id;

Let's say there is only one record in d, and it is type_a. There are two records in ta and tb each. The record in d matches one of the records in ta on d.type_id=ta.id. Therefore, that combination of (d x ta) allows ANY tb record to remain in the final result. You get an unintended cartesian product.

SELECT x.customer_id, SUM(data) total
FROM
(
    SELECT ta.customer_id, d.data
    FROM data d JOIN ta
       ON (d.type LIKE "type_a" AND d.type_id = ta.id) 
    UNION ALL
    SELECT tb.customer_id, d.data
    FROM data d JOIN tb
       ON (d.type LIKE "type_b" AND d.type_id = tb.id) 
) X
GROUP BY x.customer_id;