SQL Distinct关键字未为“多列”返回不同的值

Question

如果我只选择了一个具有Distinct的列，那么如果我选择了多个具有1个不同的列，它会返回重复的值。。

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'

返回重复的行

 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'

如果只选择Distinct Row，则效果非常好。

Answer 1

DISTINCT不能保证在与其他列一起使用时SRC_TABLE_NAME是唯一的。你的行是DISTINCT，它运行得非常好

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

将GROUP BY与aggregate function一起使用，如MIN/MAX/GROUP_CONCAT:

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;

还要尽量避免逗号语法联接，并使用JOIN。请记住，SQL Server和MySQL支持不同的聚合函数。

Answer 2

您可以这样尝试使用多列获取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，您可以像这样使用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