3.6.4 包含某列最大值的行
任务：针对每种商品，找到价格最贵的经销商。
这个问题可以用如下子查询解决：
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子查询，这可能效率低下（参阅章节 13.2.15.7，“相关子查询”）。解决该问题的其他方法是在 FROM 子句中使用不相关的子查询、LEFT JOIN 或带有窗口函数的公共表表达式。
不相关的子查询：
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，没有更大值的 s2.price，因此相应的 s2.article 值为 NULL。参阅章节 13.2.13.2，“JOIN 子句”。
具有窗口函数的公共表表达式：
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;