Collapse consecutive similar records into a single record

Question

I have records of people from an old system that I'm trying to convert over to the new system. In the old system, a person might end up with several records for the same location. They could also go from location, to another, and then return to the previous location. Here's some example data:

PersonID  | LocationID | StartDate  | EndDate
1         | 1          | 1980-07-30 | 2007-07-16
1         | 1          | 2007-07-16 | 2008-01-30
1         | 2          | 2008-01-30 | 2009-03-02
1         | 2          | 2009-03-02 | 2009-11-06
1         | 3          | 2014-07-16 | 2015-01-16
1         | 1          | 2016-01-26 | 2999-12-31

I would like to collapse this data so that I get a date range for any consecutive LocationIDs. For the data above, this is what I would expect:

PersonID  | LocationID | StartDate  | EndDate
1         | 1          | 1980-07-30 | 2008-01-30
1         | 2          | 2008-01-30 | 2009-11-06
1         | 3          | 2014-07-16 | 2015-01-16
1         | 1          | 2016-01-26 | 2999-12-31

I'm unsure as to how to do this. I previously tried joining to the previous record, but that only works when there's two consecutive locations, not with 3 or more (there could be an undefined number of consecutive records).

select
    a.PersonID,
    a.LocationID,
    a.StartDate,
    a.EndDate,
    case when a.LocationID = b.LocationID then a.PK_ID else b.PK_ID end as NewID
from employees a
left outer join employees b
on a.PersonID = b.PersonID
and a.PK_ID = b.PK_ID - 1

So, how can I write a query to get the results I need?

Note: we're treating '2999-12-31' are our 'NULL' date field

Answer 1

This is a classic Gaps-and-Islands (Edit- corrected for larger span 2999)

Select [PersonID]
      ,[LocationID]
      ,[StartDate]  = min(D)
      ,[EndDate]    = max(D)
 From (
        Select *
              ,Grp = Row_Number() over (Order By D) - Row_Number() over (Partition By [PersonID],[LocationID] Order By D) 
         from YourTable A
         Cross Apply (
                        Select Top (DateDiff(DAY,A.[StartDate],A.[EndDate])+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),A.[StartDate])  
                        From  master..spt_values n1,master..spt_values n2
                     ) B
      ) G
 Group By [PersonID],[LocationID],Grp
 Order By [PersonID],min(D)

Returns

PersonID    LocationID  StartDate   EndDate
1           1           1980-07-30  2008-01-30
1           2           2008-01-30  2009-11-06
1           3           2014-07-16  2015-01-16
1           1           2016-01-26  2999-12-31

Using your original query

Select [PersonID]
      ,[LocationID]
      ,[StartDate]  = min(D)
      ,[EndDate]    = max(D)
 From (
        Select *
              ,Grp = Row_Number() over (Order By D) - Row_Number() over (Partition By [PersonID],[LocationID] Order By D) 
         From (
                -- Your Original Query
                select
                    a.PersonID,
                    a.LocationID,
                    a.StartDate,
                    a.EndDate,
                    case when a.LocationID = b.LocationID then a.PK_ID else b.PK_ID end as NewID
                from employees a
                left outer join employees b
                on a.PersonID = b.PersonID
                and a.PK_ID = b.PK_ID - 1
              ) A
         Cross Apply (
                        Select Top (DateDiff(DAY,A.[StartDate],A.[EndDate])+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),A.[StartDate])  
                        From  master..spt_values n1,master..spt_values n2
                     ) B
      ) G
 Group By [PersonID],[LocationID],Grp
 Order By [PersonID],min(D)

Requested Comments

Let's break it down to its components.

1) The CROSS APPLY Portion: This will expand a single record into N records. For example:

Declare @YourTable Table ([PersonID] int,[LocationID] int,[StartDate] date,[EndDate] date)
Insert Into @YourTable Values
 (1,1,'1980-07-01','1980-07-03' )
,(1,1,'1980-07-02','1980-07-04' )  -- Notice the Overlap
,(1,2,'2008-01-30','2008-02-05')

Select *
    from @YourTable A
    Cross Apply (
                Select Top (DateDiff(DAY,A.[StartDate],A.[EndDate])+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),A.[StartDate])  
                From  master..spt_values n1,master..spt_values n2
                ) B

The above query will generate

2) The Grp Portion: Perhaps easier if I provide a simple example:

Declare @YourTable Table ([PersonID] int,[LocationID] int,[StartDate] date,[EndDate] date)
Insert Into @YourTable Values
 (1,1,'1980-07-01','1980-07-03' )
,(1,1,'1980-07-02','1980-07-04' )  -- Notice the Overlap
,(1,2,'2008-01-30','2008-02-05')

Select *
      ,Grp = Row_Number() over (Order By D) - Row_Number() over (Partition By [PersonID],[LocationID] Order By D) 
      ,RN1 = Row_Number() over (Order By D)
      ,RN2 = Row_Number() over (Partition By [PersonID],[LocationID] Order By D) 
    from @YourTable A
    Cross Apply (
                Select Top (DateDiff(DAY,A.[StartDate],A.[EndDate])+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),A.[StartDate])  
                From  master..spt_values n1,master..spt_values n2
                ) B

The above query Generates:

RN1 and RN2 are breakouts of the GRP, just to illustrate the mechanic. Notice RN1 minus RN2 equals the GRP. Once we have the GRP, it becomes a simple matter of aggregation via a group by

3) Pulling it all Together:

Declare @YourTable Table ([PersonID] int,[LocationID] int,[StartDate] date,[EndDate] date)
Insert Into @YourTable Values
 (1,1,'1980-07-01','1980-07-03' )
,(1,1,'1980-07-02','1980-07-04' )  -- Notice the Overlap
,(1,2,'2008-01-30','2008-02-05')

Select [PersonID]
      ,[LocationID]
      ,[StartDate]  = min(D)
      ,[EndDate]    = max(D)
 From (
        Select *
              ,Grp = Row_Number() over (Order By D) - Row_Number() over (Partition By [PersonID],[LocationID] Order By D) 
            from @YourTable A
            Cross Apply (
                        Select Top (DateDiff(DAY,A.[StartDate],A.[EndDate])+1) D=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),A.[StartDate])  
                        From  master..spt_values n1,master..spt_values n2
                        ) B
      ) G
 Group By [PersonID],[LocationID],Grp
 Order By [PersonID],min(D)

Returns

Answer 2

For your sample data, you can use the difference of row numbers approach:

select personid, locationid, min(startdate), max(enddate)
from (select e.*,
             row_number() over (partition by personid order by startdate) as seqnum_p,
             row_number() over (partition by personid, locationid order by startdate) as seqnum_pl
      from employees  e
     ) e
group by (seqnum_p - seqnum_pl), personid, locationid;

This assumes that the start and end dates are contiguous. That is, there is no gap for a given employee at the same location.