Question

I’ve got the table:

SELECT * FROM shop;

+---------+--------+------
| article | dealer | price
+---------+--------+------
|    0001 | A      |  3.45
|    0001 | B      |  3.99
|    0002 | A      | 10.99
|    0003 | B      |  1.45
|    0003 | C      |  1.69
|    0003 | D      |  1.25
|    0004 | D      | 19.95
+---------+--------+------
7 rows in set (0.20 sec)

And I want to get – for each article – the dealer or dealers with the most expensive price.

Could anyone tell me why this doesn’t work?

SELECT article, dealer, MAX(price) FROM shop GROUP BY(article);

For this query, I get the following result-set;

+---------+--------+------------+
| article | dealer | MAX(price) |
+---------+--------+------------+
|    0001 | A      |       3.99 |
|    0002 | A      |      10.99 |
|    0003 | B      |       1.69 |
|    0004 | D      |      19.95 |
+---------+--------+------------+
4 rows in set (0.03 sec)

Although the max prices are correct, I got the wrong dealers for some articles.

Answer 1

Standard SQL would reject your query because you can not SELECT non-aggregate fields that are not part of the GROUP BY clause in an aggregate query.

You’re using a MySQL extension of SQL described here:

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.