Reference price and subtract from another during group_concat in MySQL - mysql

Question

I'm working on manipulating a product data feed, and am currently working on grouping the related products. I've almost got things where I want them, but, like a mediocre racing driver, I've run out of skill right when I need it the most.
To illustrate my problem I've created a simplified version. Here is the data structure:
CREATE TABLE `feed` (
`sku` VARCHAR(10),
`price` DECIMAL(6,2),
`groupkey` VARCHAR(10)
);
INSERT INTO `feed` (`sku`, `price`, `groupkey`) VALUES
('AAA', 10.00, NULL),
('BBB', 10.00, 'group1'),
('CCC', 12.00, 'group1'),
('DDD', 10.00, 'group2'),
('EEE', 12.00, 'group2'),
('FFF', 14.00, 'group2'),
('GGG', 20.00, NULL);
My current query is:
SELECT feed.groupkey
, group_concat(feed.sku) AS skus
, group_concat(feed.price) AS prices
, feed.price AS pprice
FROM
feed
WHERE
feed.groupkey IS NOT NULL
GROUP BY
feed.groupkey;
The query returns the following rows:
+----------+-------------+-------------------+--------+
| groupkey | skus | prices | pprice |
+----------+-------------+-------------------+--------+
| group1 | BBB,CCC | 10.00,12.00 | 10.00 |
| group2 | DDD,EEE,FFF | 10.00,12.00,14.00 | 10.00 |
+----------+-------------+-------------------+--------+
What I actually need to do is subtract pprice from each concatenated price, giving me the price difference between each sku, rather than their absolute prices. This would return the dream result:
+----------+-------------+-------------------+--------+
| groupkey | skus | prices | pprice |
+----------+-------------+-------------------+--------+
| group1 | BBB,CCC | 0.00,2.00 | 10.00 |
| group2 | DDD,EEE,FFF | 0.00,2.00,4.00 | 10.00 |
+----------+-------------+-------------------+--------+
I've spent a lot of time on this feed in general, and am really stuck on what is probably the last hurdle in the integration. I'd really appreciate some guidance to help me in the right direction.
edit: I'm using the results from this query as "virtual" product rows, to serve as parents for the products in the group.

Answer 1

You can just do the subtraction in the group_concat(), for something like:
SELECT feed.groupkey, group_concat(feed.sku) AS skus,
group_concat(feed.price - min(feed.price)) AS prices
min(feed.price) AS pprice
FROM feed
WHERE feed.groupkey IS NOT NULL
GROUP BY feed.groupkey
The problem is . . . which feed.price? The value returned in your original query is an arbitrary value from one of the rows in the group. Thinking that you might want the difference over the minimum, I used that value.
I think the best way to write the query is:
SELECT feed.groupkey, group_concat(feed.sku) AS skus,
group_concat(feed.price - fsum.minprice) AS prices
min(feed.price) AS pprice
FROM feed left outer join
(select groupkey, MIN(feed.price) as minprice
from feed
group by groupkey
) fsum
on feed.groupkey = fsum.groupkey
WHERE feed.groupkey IS NOT NULL
GROUP BY feed.groupkey
You CANNOT assume the ordering for hidden columns and group_concat(). The documentation is quite explicit on this point:
MySQL extends the use of GROUP BY so that the select list can refer to
nonaggregated columns not named in the GROUP BY clause. This means
that the preceding query is legal in MySQL. You can use this feature
to get better performance by avoiding unnecessary column sorting and
grouping. However, this is useful primarily when all values in each
nonaggregated column not named in the GROUP BY are the same for each
group. The server is free to choose any value from each group, so
unless they are the same, the values chosen are indeterminate.
Furthermore, the selection of values from each group cannot be
influenced by adding an ORDER BY clause. Sorting of the result set
occurs after values have been chosen, and ORDER BY does not affect
which values the server chooses.
If you want things in a particular order, then you need to be sure the structure is queried properly. That said, it often works in practice, but there is no guarantee.