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(mysql.info)example-Maximum-column-group-row

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The Rows Holding the Group-wise Maximum of a Certain Field
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"For each article, find the dealer(s) with the most expensive price."

In ANSI SQL, I'd do it with a sub-query like this:

     SELECT article, dealer, price
     FROM   shop s1
     WHERE  price=(SELECT MAX(s2.price)
                   FROM shop s2
                   WHERE s1.article = s2.article);

In MySQL it's best do it in several steps:

  1. Get the list of (article,maxprice).

  2. For each article get the corresponding rows that have the stored
     maximum price.

This can easily be done with a temporary table:

     CREATE TEMPORARY TABLE tmp (
             article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
             price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);
     
     LOCK TABLES shop read;
     
     INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;
     
     SELECT shop.article, dealer, shop.price FROM shop, tmp
     WHERE shop.article=tmp.article AND shop.price=tmp.price;
     
     UNLOCK TABLES;
     
     DROP TABLE tmp;

If you don't use a `TEMPORARY' table, you must also lock the 'tmp'
table.

"Can it be done with a single query?"

Yes, but only by using a quite inefficient trick that I call the
"MAX-CONCAT trick":

     SELECT article,
            SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
       0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
     FROM   shop
     GROUP BY article;
     
     +---------+--------+-------+
     | article | dealer | price |
     +---------+--------+-------+
     |    0001 | B      |  3.99 |
     |    0002 | A      | 10.99 |
     |    0003 | C      |  1.69 |
     |    0004 | D      | 19.95 |
     +---------+--------+-------+

The last example can, of course, be made a bit more efficient by doing
the splitting of the concatenated column in the client.