合并数据库中的重复时间记录

Question

我有一个临时数据库表，其中某些数据是重复的。

EmployeeId   StartDate   EndDate     Column1   Column2
1000         2009/05/01  2010/04/30   X         Y
1000         2010/05/01  2011/04/30   X         Y
1000         2011/05/01  2012/04/30   X         X
1000         2012/05/01  2013/04/30   X         Y
1000         2013/05/01  NULL         X         X

如上所示，有些行是多余的，可以合并为单个行而不会违反数据有效性。我想尽可能合并这样的行，结果应该像这样

EmployeeId   StartDate   EndDate     Column1   Column2
1000         2009/05/01  2011/04/30   X         Y
1000         2011/05/01  2012/04/30   X         X
1000         2012/05/01  2013/04/30   X         Y
1000         2013/05/01  NULL         X         X

如何做到这一点？

Answer 1

如果可以确保所有开始日期和结束日期都是连续的，请尝试以下操作：

with  t1 as  --tag first row with 1 in a continuous time series
(
select t1.*, case when t1.column1=t2.column1 and t1.column2=t2.column2
                  then 0 else 1 end as tag
  from your_table t1
  left join your_table t2
    on t1.EmployeeId= t2.EmployeeId and dateadd(day,-1,t1.StartDate)= t2.EndDate
)
select t1.EmployeeId, t1.StartDate, 
       case when min(T2.StartDate) is null then null
            else dateadd(day,-1,min(T2.StartDate)) end as EndDate,
       t1.Column1, t1.Column2
  from (select t1.* from t1 where tag=1 ) as t1  -- to get StartDate
  left join (select t1.* from t1 where tag=1 ) as t2  -- to get a new EndDate
    on t1.EmployeeId= t2.EmployeeId and t1.StartDate < t2.StartDate
 group by t1.EmployeeId, t1.StartDate, t1.Column1,   t1.Column2