SQL Distinct關鍵字不為多列返回不同的值

Question

如果我只選擇1個具有Distinct的列，則工作正常，如果我選擇了1個不同的多列，則返回重復值..

SELECT
    DISTINCT MS.SRC_TABLE_NAME AS SRC_TABLE_NAME,
    MS.SRC_SYSTEM_ENVIRONMENT_NAME AS SRC_SYSTEM_ENVIRONMENT_NAME,
    MS.SRC_SYSTEM_NAME AS SRC_SYSTEM_NAME
FROM
    MAPPING_SPECIFICATION MS,
    MAPPING_DETAILS MD

WHERE
    MS.MAP_ID = MD.MAP_ID AND
    MD.STATUS = 'Active'

返回重復行

 SELECT
        DISTINCT MS.SRC_TABLE_NAME AS SRC_TABLE_NAME        
    FROM
        MAPPING_SPECIFICATION MS,
        MAPPING_DETAILS MD

    WHERE
        MS.MAP_ID = MD.MAP_ID AND
        MD.STATUS = 'Active'

如果僅選擇Distinct Row，則效果很好。

Answer 1

DISTINCT不保證您將SRC_TABLE_NAME與其他列一起使用時具有唯一性。 您的行是DISTINCT ，效果很好

CREATE TABLE #MyTable(col1 INT, col2 INT);

INSERT INTO #MyTable VALUES (1,1), (1,2), (2,1),(3,1);

SELECT DISTINCT col1, col2
FROM #MyTable;

LiveDemo

使用GROUP BY代替MIN/MAX/GROUP_CONCAT類的aggregate function ：

SELECT
    MS.SRC_TABLE_NAME AS SRC_TABLE_NAME,
    MIN(MS.SRC_SYSTEM_ENVIRONMENT_NAME) AS SRC_SYSTEM_ENVIRONMENT_NAME,
    MIN(MS.SRC_SYSTEM_NAME) AS SRC_SYSTEM_NAME
FROM MAPPING_SPECIFICATION MS
JOIN MAPPING_DETAILS MD
  ON MS.MAP_ID = MD.MAP_ID AND
WHERE MD.STATUS = 'Active'
GROUP BY MS.SRC_TABLE_NAME;

也請嘗試避免使用逗號語法JOIN並使用JOIN 。 請記住， SQL Server和MySQL支持不同的聚合函數。

Answer 2

您可以嘗試這樣獲得具有多個列的DISTINCT：

select 
(SELECT group_concat(DISTINCT MS.SRC_TABLE_NAME) FROM MAPPING_SPECIFICATION MS INNER JOIN MAPPING_DETAILS MD ON MS.MAP_ID = MD.MAP_ID 
WHERE MD.STATUS = 'Active') as SRC_TABLE_NAME,
(SELECT group_concat(DISTINCT MS.SRC_SYSTEM_ENVIRONMENT_NAME) FROM MAPPING_SPECIFICATION MS INNER JOIN MAPPING_DETAILS MD ON MS.MAP_ID = MD.MAP_ID 
WHERE MD.STATUS = 'Active') as SRC_SYSTEM_ENVIRONMENT_NAME,
(SELECT group_concat(DISTINCT MS.SRC_SYSTEM_NAM) FROM MAPPING_SPECIFICATION MS INNER JOIN MAPPING_DETAILS MD ON MS.MAP_ID = MD.MAP_ID 
WHERE MD.STATUS = 'Active') as SRC_SYSTEM_NAME

它在MYSQL中。

在SQL Server中，您可以這樣使用Group By：

SELECT MS.SRC_SYSTEM_NAM AS SRC_TABLE_NAME,
          MS.SRC_SYSTEM_ENVIRONMENT_NAME AS SRC_SYSTEM_ENVIRONMENT_NAME,
          MS.SRC_SYSTEM_NAME AS SRC_SYSTEM_NAME
   FROM MAPPING_SPECIFICATION MS 
        INNER JOIN MAPPING_DETAILS MD ON MS.MAP_ID = MD.MAP_ID 
   WHERE MD.STATUS = 'Active'
   GROUP BY MS.SRC_SYSTEM_NAM, MS.SRC_SYSTEM_ENVIRONMENT_NAME, MS.SRC_SYSTEM_NAME