Question

我正在尝试从包含国家/地区代码数据的3个不同列的表中选择不同国家/地区代码的列表。

示例

  Raw                                         Desired

+-----------+-----------+-----------+       +-----------+
| country_1 | country_2 | country_3 |       | countries |
+-----------+-----------+-----------+       +-----------+
| GBR       | GBR       | IRL       |       | ARG       |
| DEU       | FRA       | CHE       |       | AUS       |
| CHN       | CHN       | IND       |       | BRA       |
| USA       | CAN       | MEX       |       | CAN       |
| AUS       | JAP       | KOR       |       | CHE       |
| BRA       | ARG       | ARG       |       | CHN       |
| NOR       | SWE       | FIN       |       | DEU       |
| GBR       | FRA       | GBR       |       | FIN       |
| IRL       | IRL       | IRL       |       | FRA       |
+-----------+-----------+-----------+       | GBR       |
                                            | IND       |
                                            | IRL       |
                                            | JAP       |
                                            | KOR       |
                                            | MEX       |
                                            | NOR       |
                                            | SWE       |
                                            | USA       |
                                            +-----------+

我到目前为止最接近的是查询：

mysql> SELECT
        CONCAT(
            (SELECT GROUP_CONCAT(DISTINCT country_1 ORDER BY country_1) FROM tbl),
            ',',
            (SELECT GROUP_CONCAT(DISTINCT country_2 ORDER BY country_2) FROM tbl),
            ',',
            (SELECT GROUP_CONCAT(DISTINCT country_3 ORDER BY country_3) FROM tbl)
        ) AS countries;

+-------------------------------------------------------------------------------------------------+  
| countries                                                                                       |  
+-------------------------------------------------------------------------------------------------+  
| AUS,BRA,CHN,DEU,GBR,IRL,NOR,USA,ARG,CAN,CHN,FRA,GBR,IRL,JAP,SWE,ARG,CHE,FIN,GBR,IND,IRL,KOR,MEX |  
+-------------------------------------------------------------------------------------------------+

但正如你所看到的，仍然有重复，我不知道如何将它们排成行。

感谢任何帮助！

Answer 1

我不确定你为什么要使用群组连结。使用union可以轻松完成所需的结果，自动删除重复项。

SELECT country1 AS countries FROM tab
UNION
SELECT country2 FROM tab
UNION
SELECT country3 FROM tab
ORDER BY 1;

Answer 2

您得到重复项，因为您只在每列中消除重复项，而不是整体。你可能想要更像这样的东西：

SELECT GROUP_CONCAT(country)
  FROM (
    SELECT DISTINCT country1 AS country FROM tbl
    UNION
    SELECT DISTINCT country2 AS country FROM tbl
    UNION
    SELECT DISTINCT country3 AS country FROM tbl
  )

从表中选择包含类似数据的多个列的唯一值

2 个答案: