### 3.6.4 擁有某個字段的組間最大值的行 *任務: 對每個物品, 找出最貴價格物品的經銷商.* 這個問題可以通過這樣的子查詢來解決: ```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 | +---------+--------+-------+ ``` 前面的例子使用了關聯子查詢, 這可能是低效的(參閱 [Section 13.2.11.7, “關聯子查詢”](https://dev.mysql.com/doc/refman/8.0/en/correlated-subqueries.html)). 解決此問題的其它可能性是在 `FROM` 子句中使用不相關的子查詢, `LEFT JOIN`, 或者帶有窗口函數的公共表表達式. 非關聯子查詢: ```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` 的基本工作原理是當 `s1.price` 處于最高值時, 并沒有 `s2.price` 對應 `s2.article` 的值為 `NULL`. 參閱 [Section 13.2.10.2, “JOIN 語法”](https://dev.mysql.com/doc/refman/8.0/en/join.html). 使用窗口函數的公共表表達式: ```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; ```