Calculating a running total of when a value changes over a partition - sql

Question

I am having trouble figuring out how to write a window function that solves my problem. I am quite the novice at window functions, but I think one could be written to meet my needs.
Problem Statement:
I want to calculate a transfer sequence showing when person has changed locations based on the corresponding location ID over time.
Sample Data (Table1)
+----------+------------+-----------+---------+
| PersonID | LocationID | Date | Time |
+----------+------------+-----------+---------+
| 12 | A | 6/17/2020 | 12:00PM |
+----------+------------+-----------+---------+
| 12 | A | 6/18/2020 | 1:00PM |
+----------+------------+-----------+---------+
| 12 | B | 6/18/2020 | 6:00AM |
+----------+------------+-----------+---------+
| 12 | C | 6/19/2020 | 3:00PM |
+----------+------------+-----------+---------+
| 13 | A | 6/16/2020 | 8:00AM |
+----------+------------+-----------+---------+
| 13 | A | 6/16/2020 | 11:00AM |
+----------+------------+-----------+---------+
| 13 | A | 6/16/2020 | 12:00AM |
+----------+------------+-----------+---------+
| 13 | B | 6/16/2020 | 4:00PM |
+----------+------------+-----------+---------+
Expected Results
+----------+------------+-----------+---------+-------------------+
| PersonID | LocationID | Date | Time | Transfer Sequence |
+----------+------------+-----------+---------+-------------------+
| 12 | A | 6/17/2020 | 12:00PM | 1 |
+----------+------------+-----------+---------+-------------------+
| 12 | A | 6/18/2020 | 1:00PM | 1 |
+----------+------------+-----------+---------+-------------------+
| 12 | B | 6/18/2020 | 6:00AM | 2 |
+----------+------------+-----------+---------+-------------------+
| 12 | C | 6/19/2020 | 3:00PM | 3 |
+----------+------------+-----------+---------+-------------------+
| 13 | A | 6/16/2020 | 8:00AM | 1 |
+----------+------------+-----------+---------+-------------------+
| 13 | A | 6/16/2020 | 11:00AM | 1 |
+----------+------------+-----------+---------+-------------------+
| 13 | A | 6/16/2020 | 12:00AM | 1 |
+----------+------------+-----------+---------+-------------------+
| 13 | B | 6/16/2020 | 4:00PM | 2 |
+----------+------------+-----------+---------+-------------------+
What I Tried
SELECT
[t1].[PersonID]
,[t1].[LocationID]
,[t1].[Date]
,[t1].[Time]
,DENSE_RANK()
OVER(
partition BY [t1].[PersonID], [t1].[LocationID]
ORDER BY [t1].[Date] ASC, [t1].[Time] ASC) AS
[Transfer Sequence]
FROM Table1 [t1]
Unfortunately, I believe DENSE_RANK() is assigning a rank regardless of whether the value of LocationID has changed. I need a function that will only add one to the sequence when the LocationID has changed.
Any help would be greatly appreciated.
Thank you!

Answer 1

You want to put "adjacent" rows in the same group. Straigt window functions cannot do that for you - we would need to use a gaps-and-island technique:
select
t.*,
sum(case when locationID = lagLocationID then 0 else 1 end)
over(partition by personID order by date, time)
as transfert_sequence
from (
select
t.*,
lag(locationID)
over(partition by personID order by date, time)
as lagLocationID
from mytable t
) t
The idea is to compute a window sum that increments everytime the locationID changes.
Note that this would properly handle the case when a person comes back to a location they have already been before.

Answer 2

What wuold I do (and I'm sure it's not the best way) is create a second table orderd with PersonID, locationID, Date, time and and empty field for the transfer sequence (sequence), then a cursor:
DECLARE transaction CURSOR
FOR select PersonID, LocationID, Date, Time from table1;
Then a loop:
OPEN CURSOR transaction
set #count = 0
set #person_saved = ""
set #location_saed = ""
FETCH NEXT FROM transaction INTO #person, #location, #date, #time
WHILE ##FETCH_STATUS = 0
BEGIN
if #person_saved <> #person -- changing personID, reset count
begin
set count = 0
set persone_saved = #person
end
if #location_saved <> #location. -- changing location, add count
begin
set #count = #count + 1
set #location_saved = #location
end
update table1 set sequence = #count where PersonId = #person and locationId = #location and date = #date and time = #time
FETCH NEXT FROM transaction INTO #person, #location, #date, #time
END
CLOSE transaction
DEALLOCATE transaction