3.6.4 包含某列的分组最大值的行

任务：对于每件商品，寻找价格最昂贵的经销商.

这个问题可以通过像这样的子查询来解决：

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

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | C      |  1.69 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

前面的示例使用了一个关联的子查询，该子查询可能效率不高(请参见第 13.2.10.7 节“相关子查询”)。解决该问题的其他可能性是在FROM子句或LEFT JOIN中使用不相关的子查询。

Uncorrelated subquery:

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
ORDER BY article;

LEFT JOIN :

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
ORDER BY s1.article;

LEFT JOIN的工作原理是，当s1.price处于最大值时，不存在更大的s2.price值，因此相应的s2.article值为NULL。参见第 13.2.9.2 节“ JOIN 子句”。