TSQL - 使用自连接比较两列 - 计算昨天总计和今天总计之间的差异

Question

我正在尝试计算列上今天和昨天的总数之间的差异。如果自我加入不是最好的方法，那很好，无论哪个能给我我想要的结果都应该没问题。

要求：

1. 仅比较最近 2 天的数据，即使该表将包含多天的数据，但每天只有一个条目。
2. 计算该列昨天总数和今天总数之间的差值。

问题

下面的代码返回一个零，我不明白为什么。

为什么不计算，请问我该怎么做才能满足要求？

---

IF OBJECT_ID('tempdb..#t1') IS NOT NULL DROP TABLE #t1

CREATE TABLE #t1 (
 countID UNIQUEIDENTIFIER
,empCount VARCHAR(20)
,CountDate DATETIME
)

INSERT INTO #t1 (
    countID
  , empCount
  , CountDate
)
VALUES
     (NEWID(),'123000', GETDATE()) 
    ,(NEWID(),'100', '20200813')
    ,(NEWID(),'100', '20200810')

SELECT 
    today.countID
  , (CAST(today.empCount AS INT)) - (CAST(yesterday.empCount AS INT)) AS CountDiff
  , today.empCount
  , today.CountDate
FROM #t1 AS today
INNER JOIN #t1 AS yesterday ON today.countID = yesterday.countID
                                AND yesterday.CountDate > (SELECT dateadd(day,datediff(day,2,GETDATE()),0))

Answer 1

我想你想要lag():

select t.*,
       (empcount - lag(empcount) over (order by countdate)) as diff
from #t1 t;

如果你只想要最后两天，那么：

select top (1) t.*
from (select t.*,
             (empcount - lag(empcount) over (order by countdate)) as diff
      from #t1 t
     ) t
order by countdate desc;

注意：这将“昨天”解释为表格中的最后两天。如果你真的想要今天和昨天，那么你可以使用where 子句：

select top (1) t.*
from (select t.*,
             (empcount - lag(empcount) over (order by countdate)) as diff
      from #t1 t
      where countdate >= dateadd(day, -1, convert(date, getdate()))
     ) t
order by countdate desc;