选择Count（Distinct Value）返回1

Question

我正在SSMS 2005中设计一个查询，如下所示：

SELECT COUNT(DISTINCT ColumnName) FROM Table WHERE ColumnName IS NOT NULL

当我使用COUNT（）运行查询时，它返回值1.当我在没有COUNT（）的情况下运行它时，SSMS会报告正确的值，例如212条记录。

有问题的列是数据类型numeric（16,0）。

对于那些可能会问的人，查询完整是：

SELECT COUNT(DISTINCT O_ID) FROM vEmployers
INNER JOIN vEnrolment ON O_ID = E_EnrolmentEmployer
WHERE E_START >= '01-AUG-2008' AND E_START < '01-AUG-2009'
AND O_ID IS NOT NULL AND O_ID IN (
    SELECT O_ID FROM vEmployers
    INNER JOIN vEnrolment ON O_ID = E_EnrolmentEmployer
    WHERE E_Start < '01-AUG-2008' and E_Start >= '01-AUG-2007'
)

此查询基本上给出了两个12个月之间的重复业务数据。

所以我想知道为什么“COUNT（DISTINCT ColumnName）”在指定“ColumnName IS NOT NULL”时返回1？

以下是运行SELECT TOP 10 DISTINCT ColumnName FROM ...等时的数据示例：

1346116
1346131
1346425
1346923
1349935
1350115
1350153
2594787
2821944
2879631

Answer 1

使用数字（16,0）让我怀疑它与数据类型有关。在COUNT子句中添加CAST以将其强制转换为INT类型：

Count(Distinct Cast(O_ID as Int))

Answer 2

我猜是因为返回的所有行共享O_ID的相同值。您可以对每行唯一的密钥执行COUNT(*)或COUNT()以获取行数。

Answer 3

删除DISTINCT，您将获得所有行的计数。

Answer 4

请你运行这些查询：

SELECT  COUNT(DISTINCT O_ID)
FROM    vEmployers
INNER JOIN
        vEnrolment
ON      O_ID = E_EnrolmentEmployer
WHERE   E_START >= '01-AUG-2008' AND
        E_START < '01-AUG-2009'
        AND O_ID IN
        (
        SELECT  O_ID
        FROM    vEmployers
        INNER JOIN
                vEnrolment
        ON      O_ID = E_EnrolmentEmployer
        WHERE   E_Start < '01-AUG-2008'
                AND E_Start >= '01-AUG-2007'
        )

和

SELECT  DISTINCT TOP 5 O_ID
FROM    vEmployers
INNER JOIN
        vEnrolment
ON      O_ID = E_EnrolmentEmployer
WHERE   E_START >= '01-AUG-2008' AND
        E_START < '01-AUG-2009'
        AND O_ID IN
        (
        SELECT  O_ID
        FROM    vEmployers
        INNER JOIN
                vEnrolment
        ON      O_ID = E_EnrolmentEmployer
        WHERE   E_Start < '01-AUG-2008'
                AND E_Start >= '01-AUG-2007'
        )
ORDER BY
        O_ID

逐字，没有改变什么？

Answer 5

SELECT   
  COUNT(*)  
FROM    vEmployers  
INNER JOIN vEnrolment ON O_ID = E_EnrolmentEmployer
WHERE 
        E_START >= '01-AUG-2008' 
        AND E_START < '01-AUG-2009'
        AND O_ID IS NOT NULL AND O_ID IN (
          SELECT O_ID FROM vEmployers
          INNER JOIN vEnrolment ON O_ID = E_EnrolmentEmployer
          WHERE E_Start < '01-AUG-2008' and E_Start >= '01-AUG-2007'
        )
GROUP BY
  O_Id