Task: For each article, find the dealer or dealers with the most expensive price.
In standard SQL (and as of MySQL 4.1), the problem can be solved with a subquery like this:
SELECT article, dealer, price FROM shop s1 WHERE price=(SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article);
The preceding example uses a correlated subquery, which can be
inefficient (see Section 184.108.40.206, “Correlated Subqueries”). Other
possibilities for solving the problem are to use an uncorrelated
subquery in the
FROM clause or a
SELECT s1.article, dealer, s1.price FROM shop s1 JOIN ( SELECT article, MAX(price) AS price FROM shop GROUP BY article) AS s2 ON s1.article = s2.article AND s1.price = s2.price; SELECT s1.article, s1.dealer, s1.price FROM shop s1 LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price WHERE s2.article IS NULL;
LEFT JOIN works on the basis that when
s1.price is at its maximum value, there is no
s2.price with a greater value and the
s2 rows values will be
NULL. See Section 220.127.116.11, “
Before MySQL 4.1, subqueries are unavailable. Another approach is to solve the problem in several steps:
Get the list of (article,maxprice) pairs.
For each article, get the corresponding rows that have the stored maximum price.
This can easily be done with a temporary table and a join:
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
“Can it be done with a single query?”
Yes, but only by using a quite inefficient trick called 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 be made a bit more efficient by doing the splitting of the concatenated column in the client.