如何在 Order by 子句中使用计算别名？

Question

表格很简单：

ID   start_date    end_date
1    2015-10-01    2015-10-02
2    2015-10-02    2015-10-03
3    2015-10-05    2015-10-06
4    2015-10-07    2015-10-08

ID 1 和 2 属于一个项目，因为 end_date 等于 start_date，ID 3 和 4 是不同的。

这是查找相同项目并按其花费时间排序的查询：

select P1.Start_Date, (
    select min(P.End_Date)
    from Projects  as P
    where P.End_Date not in (select Start_Date from Projects )
        and P.End_Date > P1.Start_Date
) as ED
from Projects as P1
where P1.Start_Date not in (select End_Date from Projects )
order by datediff(day, P1.Start_Date, ED)

问题是：order by子句中的ED无效，但是不带datediff使用时有效：

order by ED

datediff 是在 select 子句之后计算的吗？有谁能解释一下吗？谢谢。

Answer 1

您可以简单地使用CROSS APPLY 来计算此列，如下所示：

DECLARE @Projects TABLE
(
    [ID] SMALLINT
   ,[start_date] DATETIME
   ,[end_date] DATETIME
);

INSERT INTO @Projects ([ID], [start_date], [end_date])
VALUES ('1', '2015-10-01', '2015-10-02')
      ,('2', '2015-10-02', '2015-10-03')
      ,('3', '2015-10-05', '2015-10-06')
      ,('4', '2015-10-07', '2015-10-08');

select P1.Start_Date, ED
from @Projects as P1
CROSS APPLY
(
    select min(P.End_Date)
    from @Projects  as P
    where P.End_Date not in (select Start_Date from @Projects )
        and P.End_Date > P1.Start_Date
) DS(ED)
where P1.Start_Date not in (select End_Date from @Projects )
order by datediff(day, P1.Start_Date, ED);

管理工作室的引擎似乎无法将别名ED 转换为有效的东西。例如，如果您将ED 替换为子查询，它将起作用。此外，以下是一种不好的做法将起作用：

select P1.Start_Date, (
    select min(P.End_Date)
    from @Projects  as P
    where P.End_Date not in (select Start_Date from @Projects )
        and P.End_Date > P1.Start_Date
) as ED
from @Projects as P1
where P1.Start_Date not in (select End_Date from @Projects )
order by datediff(day, P1.Start_Date, 2)

而不是alias，我们使用要排序的列号。所以，你的代码没有问题。