SQL - 如何在 SQL 中仅按列显示一次分组

Question

我有 3 列 表格中的国家、州、城市。

我想要带有国家和州的城市列表。

我想针对具有多个城市的州显示单个国家/地区名称之类的结果。

我确实按国家和州分组，每行显示所有国家和州。但我想要 只有这样一个国家名称和州名

Country  State        City
--------------------------------
India    Maharashtra  Mumbai
                      Pune
                      Nagpur
         Gujrat       Surat
                      ahmedabad
USA      New york     Albany
                      New york City
         california   Los angles

Answer 1

你试过了吗：

SELECT DISTINCT country, state, city FROM table
GROUP BY country ASC, state ASC
ORDER BY country ASC, state ASC, city ASC;

这将只返回每列的不同值。

Answer 2

正如@Gordon 所说，最好在获取数据后在应用层执行此操作。

但如果你真的想在 sql 中这样做，这里有一个解决方案。

;WITH All_Data AS (

    SELECT 
          COUNTRY,
          STATE,
          CITY
    FROM 
          TABLE_NAME
    GROUP BY 
          COUNTRY,
          STATE,
          CITY
--which gives all countries, states and cities.
),
Grouped_by_Country AS (

    SELECT 
          *,
           ROW_NUMBER () OVER(PARTITION BY COUNTRY ORDER BY STATE ) AS COUNTRY_NO
    FROM 
         All_Data

),
Grouped_by_State AS (

    SELECT 
          *,
          ROW_NUMBER() OVER(PARTITION BY COUNTRY,STATE ORDER BY CITY ) AS STATE_NO
    FROM 
         Grouped_by_Country

) ,

Result AS (

    SELECT 
          CASE WHEN COUNTRY_NO = 1
               THEN COUNTRY
               ELSE ''
          END AS COUNTRY,

          CASE WHEN STATE_NO = 1
               THEN STATE
               ELSE ''
          END AS STATE,

          CITY,
          COUNTRY_NO,
          STATE_NO

)

SELECT 
      COUNTRY,
      STATE,
      CITY

FROM 
      Result 
ORDER BY 
      COUNTRY_NO,
      STATE_NO