SQL Server：历史表中一行中的新旧值

Question

有一个历史表，比如

id  user    vendor      createdtime 
1   sree    a           12/29/2014 23:12:21
1   sree    a           12/29/2014 23:13:21
1   sree    b           12/29/2014 23:14:21
1   jack    a           12/29/2014 23:14:21
1   jack    a           12/29/2014 23:11:21
1   sree    a           12/29/2014 23:11:21 

并需要像

这样的结果

id      user        old vendor      new vendor      createdtime
1       sree                        a               12/29/2014 23:12:21
1       sree        a               a               12/29/2014 23:13:21
1       sree        a               b               12/29/2014 23:14:21
1       jack        b               a               12/29/2014 23:14:21
1       jack        a               a               12/29/2014 23:11:21
1       sree        a               a               12/29/2014 23:11:21

Answer 1

select h.id as id , h.[user] as [user],
(select top 1 h2.vendor 
         from History h2 
         where h2.[user] = h.[user] and h2.createdtime < h.createdtime
         order by h2.createdtime desc ) as [old vendor],
     h.vendor as [new vendor]

     from History h 
     order by h.createdtime

Answer 2

在SQL Server 2012+中，您可以使用lag()功能：

select h.*, lag(vendor) over (partition by user order by createdtime) as prev_vendor
from history h;

在早期版本中，您可以使用子查询执行此操作。但是，outer apply更强大：

select h.*, hprev.vendor as prev_vendor
from history h outer apply
     (select top 1 h2.*
      from history h2
      where h2.vendor = h.vendor
      order by createdtime
     ) hprev;

这允许您从上一条记录中提取更多列 - 例如，不仅是供应商名称，还包括createdtime。