从具有包含相似数据的多列的表中选择唯一值

Question

我正在尝试从具有3个包含国家代码数据的不同列的表中选择不同国家代码的列表。

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这样的查询：

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

但是，正如您所看到的，仍然有重复项，而且我不确定如何将它们分成行。

任何帮助表示赞赏！

Answer 1

我不确定为什么要使用群组连接。 联合可以轻松实现期望的结果，联合会自动删除重复项。

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

Answer 2

您得到重复项是因为您仅在每一列中消除了重复项，而不是整体。 您可能想要更多这样的东西：

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
  )