查看“Mysql常见查询示例”的源代码

以下是使用MySQL解决一些常见问题的示例。

其中一些示例使用表格shop来存储每个交易者（dealer）的每个商品（item number）的价格。假设每个交易者对于每个商品都有一个固定的价格，那么（article，dealer）是记录的主键。

启动命令行工具mysql并选择一个数据库：

 $> mysql your-database-name
使用以下语句创建并填充示例表格：

 CREATE TABLE shop (
 article INT UNSIGNED DEFAULT '0000' NOT NULL,
 dealer CHAR(20) DEFAULT '' NOT NULL,
 price DECIMAL(16,2) DEFAULT '0.00' NOT NULL,
 PRIMARY KEY(article, dealer));
 INSERT INTO shop VALUES
 (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
 (3,'C',1.69),(3,'D',1.25),(4,'D',19.95);
执行完语句后，表格应该有以下内容：

 SELECT * FROM shop ORDER BY article;

 +---------+--------+-------+
 | article | dealer | price |
 +---------+--------+-------+
 | 1 | A | 3.45 |
 | 1 | B | 3.99 |
 | 2 | A | 10.99 |
 | 3 | B | 1.45 |
 | 3 | C | 1.69 |
 | 3 | D | 1.25 |
 | 4 | D | 19.95 |
 +---------+--------+-------+

===查询列的最大值===
 SELECT MAX(article) AS article FROM shop;
 
 +---------+
 | article |
 +---------+
 | 4 |
 +---------+


===找到某一列最大值所在行===
这可以通过子查询轻松完成：

 SELECT article, dealer, price
 FROM shop
 WHERE price=(SELECT MAX(price) FROM shop);
 
 +---------+--------+-------+
 | article | dealer | price |
 +---------+--------+-------+
 | 0004 | D | 19.95 |
 +---------+--------+-------+

另一种解决方案是使用 LEFT JOIN，如下所示：

 SELECT s1.article, s1.dealer, s1.price
 FROM shop s1
 LEFT JOIN shop s2 ON s1.price < s2.price
 WHERE s2.article IS NULL;

你也可以通过按价格降序排序所有行，并使用 MySQL 特定的 LIMIT 子句仅获取第一行，像这样：

 SELECT article, dealer, price
 FROM shop
 ORDER BY price DESC
 LIMIT 1;

 注意
 
 如果有几个价格都是 19.95 的最贵商品，则 LIMIT 解决方案只会显示其中一个。


==找到每个组的最大列值===
 SELECT article, MAX(price) AS price
 FROM shop
 GROUP BY article
 ORDER BY article;
 
 +---------+-------+
 | article | price |
 +---------+-------+
 | 0001 | 3.99 |
 | 0002 | 10.99 |
 | 0003 | 1.69 |
 | 0004 | 19.95 |
 +---------+-------+


===找到持有某一列分组最大值的行===

可以使用以下子查询解决此问题：

 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 |
 +---------+--------+-------+

上述示例使用了相关子查询，这可能效率不高。解决该问题的其他可能性是在FROM子句中使用非相关子查询、LEFT JOIN或带有窗口函数的公共表达式。


非相关子查询：

 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基于这样的事实：当s1.price达到最大值时，没有s2.price的值更大，因此相应的s2.article值为NULL。


带有窗口函数的公共表达式：

 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