Question

表很简单：

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是不同的项目。

以下是用于查询相同项目并按时间进行排序的查询：

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：ED在order by子句中无效，但在不使用datediff的情况下有效：

order by ED

在select子句之后计算datediff吗？有人可以解释吗？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转换为有效的名称。例如，如果用子查询替换ED，它将起作用。另外，下面的一种不好的做法也会起作用：

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)

相反，我们使用alias来排序的列号。因此，您的代码没有错。

如何在Order by子句中使用计算出的别名？

1 个答案: