SQL Distinct关键字不为多列返回不同的值

Question

如果我只选择1个具有Distinct的列，则工作正常，如果我选择了1个不同的多列，则返回重复值..

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'

返回重复行

 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'

如果仅选择Distinct Row，则效果很好。

Answer 1

DISTINCT不保证您将SRC_TABLE_NAME与其他列一起使用时具有唯一性。 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类的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;

也请尝试避免使用逗号语法JOIN并使用JOIN 。 请记住， SQL Server和MySQL支持不同的聚合函数。

Answer 2

您可以尝试这样获得具有多个列的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，您可以这样使用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