SQL - 使用选择查询求和

Question

I want to sum a values that came from selected statement.我想对来自选定语句的值求和。 My query goes like:我的查询是这样的：

 select sum((SELECT DATEDIFF(DAY, ActualCompletionDate, ActualStartDate) AS DateDiff))
 from ProjectMilestone;

But this query give an error但是这个查询给出了一个错误

Cannot perform an aggregate function on an expression containing an aggregate or a subquery.无法对包含聚合或子查询的表达式执行聚合函数。

How can I solve this issue?我该如何解决这个问题？

Answer 1

You don't need to use a subquery, aggregation with a single query is enough:您不需要使用子查询，使用单个查询聚合就足够了：

SELECT 
    SUM(DATEDIFF(DAY, ActualCompletionDate, ActualStartDate))
FROM
    ProjectMilestone PM;