Question

假设我有以下内容：

表A

id (auto increment primary key) | age | country
1 | 5 | Germany
2 | 6 | Spain
3 | 7 | France
4 | 7 | Spain
5 | 8 | France
6 | 9 | France
7 | 2 | Mexico
8 | 7 | Saudi Arabia
9 | 5 | Saudi Arabia

我想要结果表：

表A

id (auto increment primary key) | age | country | country_number
1 | 5 | Germany | 1
2 | 6 | Spain | 2
3 | 7 | France | 3
4 | 7 | Spain | 2
5 | 8 | France | 3
6 | 9 | France | 3
7 | 2 | Mexico | 4
8 | 7 | Saudi Arabia | 5
9 | 5 | Saudi Arabia | 5

请注意，“country_number”只是一个基于唯一国家/地区名称的唯一编号。

使用光标很慢。我如何做到这样我可以做某种“分组”？为每个国家/地区分配唯一标识符？或者没有必要分组？

Answer 1

查询可能是（沙特阿拉伯不会有5个，但每个国家至少有不同的country_number）

select a.id, a.age, a.country, b.minId as country_number
FROM TableA a
INNER JOIN (SELECT MIN(id) as minId, country
            FROM TableA
            GROUP BY country) as b
      ON a.country = b.country

SqlFiddle

要正确地做事，

CREATE TABLE COUNTRY(id int PRIMARY KEY auto_increment, name VARCHAR(100))
SELECT distinct COUNTRY as name FROM TableB;

ALTER TABLE TableB Add column countryID int NOT NULL DEFAULT 0;

UPDATE TableB b, country c
set b.countryID = c.Id
where b.country = c.name;

ALTER TABLE TableB drop column country;

ALTER TABLE TableB add  FOREIGN KEY(countryID) REFERENCES country(id);

请参阅SqlFiddle

Answer 2

您应创建一个单独的表来存储国家/地区，然后将国家/地区的主键（country_id）存储为当前表中的外键。

如何在MySQL中分组？

2 个答案: