### 3.6.4 拥有某个字段的组间最大值的行 *任务: 对每个物品, 找出最贵价格物品的经销商.* 这个问题可以通过这样的子查询来解决: ```sql 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 | +---------+--------+-------+ ``` 前面的例子使用了关联子查询, 这可能是低效的(参阅 [Section 13.2.11.7, “关联子查询”](https://dev.mysql.com/doc/refman/8.0/en/correlated-subqueries.html)). 解决此问题的其它可能性是在 `FROM` 子句中使用不相关的子查询, `LEFT JOIN`, 或者带有窗口函数的公共表表达式. 非关联子查询: ```sql 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`: ```sql 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` 的基本工作原理是当 `s1.price` 处于最高值时, 并没有 `s2.price` 对应 `s2.article` 的值为 `NULL`. 参阅 [Section 13.2.10.2, “JOIN 语法”](https://dev.mysql.com/doc/refman/8.0/en/join.html). 使用窗口函数的公共表表达式: ```sql 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; ```