如何从两个select语句中减去值

Question

我想从一个值中减去另一个值。表的模式如下：

   tag, datetime,value
   ------------
   tag1, 2010-1-1 10:10:00, 123
   tag2, 2010-2-2 10:12:00. 321

  select * from  

  ( (Select    Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10'       and tagname ='tag1') as v1   -

  (  (Select    Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2')  as v2))

显然I‘m lost...how do I do this。

谢谢

ms-sql

Answer 1

总猜测：

select v1.Value1 - v2.Value2 from  

  (Select    Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10' and tagname ='tag1') as v1   

CROSS JOIN

  (  (Select    Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2')  as v2)

Answer 2

你真的需要包装select语句吗？

您可以声明两个变量@value1和@value2并将其子结构。

declare @value1 int, @value2 int

select @value1 = Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10'       and tagname ='tag1'

select @value2 = Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2'

select @value1 - @value2

Answer 3

值列的类型是什么？如果它已经是一个整数，只需执行以下操作：

SELECT 

  (Select Max(Value) as [Value1] from History WHERE Datetime ='2010-1-1 10:10'       and tagname ='tag1') as v1   -

  (Select Max(Value) as [Value2] from History WHERE Datetime ='2010-1-1 10:12'      and Tagname ='tag2')  as v2

否则，您将不得不将其转换为整数或任何您想要的数值类型