T-SQL：如何計算值的總和

Question

我想在T-SQL中執行計算，但遇到了一些問題。 這裏是我想要做的事：

DECLARE @CNT money 
SELECT @CNT = 0 

Select Amount, 
     case 
      when Service like 'pay_in' then SET @CNT = @CNT + Amount 
      when Service like 'pay_out' then SET @CNT= @CNT - Amount 
     end 
    from Payment where isActive = 1 

select @CNT 

由於我的T-SQL語法，我呆在這裏的知識貧乏，而且會非常感激，如果有人可以把我在正確的方向。謝謝！

Answer 1

SQL最適合與基於集合的方法。儘量不要通過迭代數據集來解決問題，而是通過聚合它來解決問題。

DECLARE @CNT money 

SELECT 
    @CNT = SUM(
    CASE Service 
     WHEN 'pay_in' THEN Amount 
     WHEN 'pay_out' THEN -1 * Amount 
     ELSE 0 
    END 
) 
FROM 
    Payment 
WHERE 
    isActive = 1 

SELECT @CNT 

Answer 2

可以進一步簡化它，而不使用變量：

Select Sum(Case Service 
      When 'pay_in' Then Amount 
      When 'pay_out' Then -Amount 
      End) as Total 
From Payment 
Where isActive = 1 

Answer 3

沒有必要爲SET一個SELECT內。嘗試：

set @CNT = 0 

select 
    @CNT = case 
     when Service like 'pay_in' then @CNT + Amount 
     when Service like 'pay_out' then @CNT - Amount 
    end 
from Payment 
where isActive = 1 

或者，沒有一個變量：

Select 
    sum(case 
     when Service like 'pay_in' then Amount 
     when Service like 'pay_out' then -Amount 
    end) as Total 
from Payment 
where isActive = 1