Question

我尝试在SAME表上使用嵌套的SELECT进行SQL查询（表中的UID是多态的）。

问题是我的嵌套SELECT总是返回NULL。

以下是查询：

SELECT
    Ent.UID,
    Measurement.MeasurementClass AS Type,
    Substation.TG8000_Name AS Station,
    SUBSTRING(Measurement.TG8000_Name,1,5) AS Travee,
    SUBSTRING(Measurement.TG8000_Name,6,8) AS Equipement,
    Measurement.ClonedFromMeasurement,
    (SELECT TOP 1 TG8000_Name 
        FROM [BD_Sonel_PTG].[dbo].[Measurement]
        WHERE (UID=Measurement.ClonedFromMeasurement) )  AS Template,
    LongName.LongName AS Description,
    Measurement.MeasurementUnit AS Units,
    MeasurementLimit.LowLimit,
    MeasurementLimit.HighLimit,
    RTU.TG8000_Name AS RTUName,
    RTU.RTUProtocol,
    RTU.Configuration AS RTUConfiguration,
    Telemetry.Address,
    Measurement.DisplayName AS Display,
    REPLACE(Measurement.MeasurementClass,' ','')+':'+REPLACE(Substation.TG8000_Name,' ','')+'.'+REPLACE(Measurement.TG8000_Name,' ','') AS Tren

  FROM EntityName Ent

  LEFT JOIN LongName ON Ent.UID = Longname.UID  /* chaque point possède une description pour une langue donnée */
  LEFT JOIN Measurement ON Ent.UID = Measurement.UID /* chaque point de type measurement possède des infos suppl dansla table Measurement */
  LEFT JOIN Substation ON Measurement.Substation = Substation.UID /* chaque point est lié à l'UID d'une station (Table Substations) */
  LEFT JOIN Telemetry ON Telemetry.DataSourceFor = Ent.UID /* 1 point télémétré est lié à une entrée de Telemetry (DataSourceFor) */
  LEFT JOIN MeasurementLimit ON Ent.UID = MeasurementLimit.UID /* Chaque point de type measurement est lié à des paramètres limites */
  LEFT JOIN RTU ON Telemetry.RTU = RTU.UID /* chaque point télémétré est associé à un l'UID d'un RTU dont les paramètres sont dans la table RTU */
 /* LEFT JOIN EntityName Ent2 ON Ent.UID = Measurement.ClonedFromMeasurement*/

  WHERE Ent.EntityType = 'Measurement'

  ORDER BY Substation.TG8000_Name, Measurement.TG8000_Name;

  GO

当我使用硬编码值9616执行嵌套选择时，它可以工作！

... (SELECT TOP 1 TG8000_Name 
        FROM [BD_Sonel_PTG].[dbo].[Measurement]
        WHERE (UID='9616') )  AS Template,
...

所以我想这是对Measurement.ClonedFromMeasurement的访问不起作用......

但正如您在查询中看到的那样，我还输出了Measurement.ClonedFromMeasurement的值以查看其中的内容，并且我正确地接收了我的9616值（从一行到另一行）。

如何在嵌套查询中访问Measurement.ClonedFromMeasurement？

Answer 1

尝试为此

进行自我加入

(SELECT TOP 1 TG8000_Name
FROM [BD_Sonel_PTG].[dbo].[Measurement] m1,m2 on
m1.UID = m2.ClonedFromMeasurement) AS Template

OR

(SELECT TOP 1 TG8000_Name
FROM [BD_Sonel_PTG].[dbo].[Measurement] 
where UID in (select distinct ClonedFromMeasurement from 
[BD_Sonel_PTG].[dbo].[Measurement]) AS Template

Answer 2

“UID”是INT列吗？如果是这样，请尝试使用

UID=Convert(int,Measurement.ClonedFromMeasurement)

子句中的

。

嵌套SELECT在Transact SQL中返回NULL

2 个答案: