任务： 查找每种商品价格最贵的经销商。

可以使用如下所示的子查询解决此问题

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子查询，这可能会效率低下（请参阅 第 15.2.15.7 节“关联子查询”）。 解决该问题的其他可能性是在 FROM 子句中使用非关联子查询、LEFT JOIN 或带有窗口函数的公用表表达式。

非关联子查询

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 处于其最大值时，没有 s2.price 具有更大的值，因此相应的 s2.article 值为 NULL。 请参阅 第 15.2.13.2 节“JOIN 子句”。

带有窗口函数的公用表表达式

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;