3.6.4 查询某一列的分组最大值的行

任务：对于每件商品，找到价格最贵的经销商或经销商。

这个问题可以用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 |
+---------+--------+-------+

前面的示例使用了相关的子查询，这可能是低效的（请参阅第 13.2.11.7 节，“相关子查询”）。解决该问题的其他可能性是在 FROM 子句中使用不相关的子查询、LEFT JOIN 或带有窗口函数的公用表表达式。

不相关子查询：

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，没有更大的 s2.price，因此对应的 s2.article 值为 NULL。

带窗函数的常用表表达式：

WITH s1 AS (
   SELECT article, dealer, price,
          RANK() OVER (PARTITION BY article
                           ORDER BY price DESC
                      ) AS `Rank`
     FROM shop
)
SELECT article, dealer, price
  FROM s1
  WHERE `Rank` = 1
ORDER BY article;