列名中的动态 SQL 以获取该列的最大值

Question

我有以下查询，我想获取在动态 SQL 中创建的列的最大值。但结果并不相同。你能帮我解决这个问题吗？

DECLARE @RT VARCHAR(6)
SET @RT     = 'RT1401'

SELECT      max(KWh_RT1401A_BLU)    AS Test1,
            max('KWh_'+@RT+'A_BLU') AS Test2

            FROM [PlantData].[dbo].[POD14_Kwh]
            where timestamp >='2017-08-24'

结果是：

Test1         Test2
------------- ---------------
730.3399      KWh_RT1401A_BLU

Answer 1

你需要像下面这样的动态 sql

DECLARE @RT VARCHAR(6)
SET @RT     = 'RT1401'
DECLARE @SQL VARCHAR(MAX)

SET @SQL=' SELECT max(KWh_RT1401A_BLU) AS Test1, '+
         ' max(KWh_'+@RT+'A_BLU) AS Test2 '+
         ' FROM [PlantData].[dbo].[POD14_Kwh] '+
         ' where timestamp >=''2017-08-24'''
EXEC( @SQL)

See working demo

根据您的评论

Now, I have bigger problem, I can't pass datetime into my dynamic SQL. I also try Cast / Convert, in triple quote but still has issue

修改后的解决方案如下

CREATE TABLE [POD14_Kwh]( [KWh_RT1401A_BLU] int, timestamp datetime)
INSERT INTO POD14_Kwh VALUES
(730.3399,'2017-08-24 00:00:00'),(1.00,'2017-08-24 00:00:00');

DECLARE @RT VARCHAR(6)
SET @RT     = 'RT1401'
DECLARE @SQL VARCHAR(MAX)
DECLARE @dt DATETIME
SET @dt = '2017-08-24 00:00:00'



SET @SQL=' SELECT max(KWh_RT1401A_BLU) AS Test1, '+
         ' max(KWh_'+@RT+'A_BLU) AS Test2 '+
         ' FROM [POD14_Kwh] '+
         ' where timestamp >='''+CAST(@dt as varchar(max))+''''
EXEC( @SQL)

demo link

Answer 2

Dhruv Joshi 是正确的，您需要使用动态 SQL。在您的示例中，您创建了一个字符串而不是引用了一个列名。确保执行动态 SQL 字符串也能收到所需的输出。

DECLARE @RT VARCHAR(6)
        ,@SQL VARCHAR(MAX)
SET @RT     = 'RT1401'


SET @SQL=' SELECT max(KWh_RT1401A_BLU) AS Test1, '+
         ' max(KWh_'+@RT+'A_BLU) AS Test2 '+
         ' FROM [PlantData].[dbo].[POD14_Kwh] '+
         ' where timestamp >= ''2017-08-24'''

EXEC (@SQL)

列名中的动态 SQL 以获取该列的最大值

Dynamic SQL in column name to get max of that column

sql

tsql

sql-server

dynamic-sql