sum（c1）* sum（c2）和sum（c1 * c2）之间的SQL不同

Question

在SQL语言中，

之间的结果或性能有何不同

sum(c1) * sum(c2) from t

和

sum(c1 * c2) from t

我用它来计算销售总额

select sum(price * quantity) as total from Bill

那么什么是最佳选择？

Answer 1

垂直和水平求和......假设没有NULLS

declare @mytable table (a int, b int)

insert into @mytable 
values 
(1,1),
(2,3)


select * from @mytable

select sum(a) * sum(b) from @mytable  -- result is 12 .. vertical/column, summarize column first then multiply to other column

select sum(a*b) from @mytable  --- result is 7  .. horizontal/row, summarize the product of a and b

Answer 2

这不是SQL的问题。这只是基本的数学。假设c1 =（0,1）且c2 =（2,3）。然后：

sum(c1)*sum(c2) alias... (0+1)*(2+3) = 5

不与：

相同

sum(c1*c2)      alias... (0*2)+(1*3) = 3

Answer 3

NULL和零是一个问题

Declare @YourTable table (C1 money,C2 money)
Insert Into @YourTable values
 (100,10)
,(100,null)

Select sum(c1) * sum(c2)   -- 2000.00
 From @YourTable

Select sum(c1 * c2)        -- 1000.00
 From @YourTable