T-SQL:如何计算总值

Question

我正在尝试在T-SQL中执行计算,但我遇到了一些问题.这是我正在尝试做的事情:

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

由于我对T-SQL语法知之甚少,我被困在这里,如果有人能够把我推向正确的方向,我将非常感激.谢谢!

Answer 1

SQL最适合基于集合的方法.尽量不要通过迭代数据集来解决问题,而是通过聚合来解决问题.

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