How to loop through a table and look for adjacent rows with identical values in one field and update another column conditionally in SQL? - sql

Question

I have a table that has a field called ‘group_quartile’ which uses the sql ntile() function to calculate which quartile does each customer lie in on the basis of their activity scores. However using this ntile(0 function i find there are some customers which have same activity scores but are in different quartiles. I need to modify the ‘group-quartile’ column to make all customers with the same activity scores lie in the same group_quartile.
A view of the table values :
Customer_id Product Activity_Score Group_Quartile
CH002 T 2328 1
CR001 T 268 1
CN001 T 178 1
MS006 T 45 2
ST001 T 21 2
CH001 T 0 2
CX001 T 0 3
KH001 T 0 3
MH002 T 0 4
SJ003 T 0 4
CN001 S 439 1
AC002 S 177 1
SC001 S 91 2
PV001 S 69 3
TS001 S 0 4
I used CTE expression but it didnot work.
My query only updates(from the above example) :
CX001 T 0 3
modified to
CX001 T 0 2
So only the first repeating activity score is checked and that row’s group_quartile is updated to 2.
I need to update all the below rows as well.
CX001 T 0 3
KH001 T 0 3
MH002 T 0 4
SJ003 T 0 4
I cannot use DENSE_RANK() instead of quartile to segregate the records as arranging the customers per product in approximately 4 quartiels is a business requirement.
From my understanding I need to loop through the table -
Find a row which has same activity score and the same product as its predecessor but has a different group_quartile
Update the selected row's group_quartile to its predecessor's quartile value
Then againg loop through the updated table to look for any row with the above condition , and update that row similarly.
The loop continues until all rows with same activity scores (for the same product) are put in the same group_quartile.
--
THIS IS THE TABLE STRUCTURE I AM WORKING ON:
CREATE TABLE #custs
(
customer_id NVARCHAR(50),
PRODUCT NVARCHAR(50),
ACTIVITYSCORE INT,
GROUP_QUARTILE INT,
RANKED int,
rownum int
)
INSERT INTO #custs
-- adding a column to give row numbers(unique id) for each row
SELECT customer_id, PRODUCT, ACTIVITYSCORE,GROUP_QUARTILE,RANKED,
Row_Number() OVER(partition by product ORDER BY activityscore desc) N
FROM
-- rows derived form a parent table based on 'segmentation' column value
(SELECT customer_id, PRODUCT, ACTIVITYSCORE,
DENSE_RANK() OVER (PARTITION BY PRODUCT ORDER BY ACTIVITYSCORE DESC) AS RANKED,
NTILE(4) OVER(PARTITION BY PRODUCT ORDER BY ACTIVITYSCORE DESC) AS GROUP_QUARTILE
FROM #parent_score_table WHERE (SEGMENTATION = 'Large')
) as temp
ORDER BY PRODUCT
The method I used to achieve this partially is as follows :
-- The query find the rows which have activity score same as its previous row but has a different GRoup_Quartiel value.
-- I need to use a query to update this row.
-- Next, find any rows in this newly updated table that has activity score same as its previous row but a differnet group_quartile vale.
-- Continue to update the tabel in the above manner until all rows with same activity scores have been updated to have the same quartile value
I managed to find only the rows which have activity score same as its previous row but has a different Group_Quartill value but cannot loop thorugh to find new rows that may match this updated row.
select t1.customer_id,t1.ACTIVITYSCORE,t1.PRODUCT, t1.RANKED, t1.GROUP_QUARTILE, t2.GROUP_QUARTILE as modified_quartile
from #custs t1, #custs t2
where (
t1.rownum = t2.rownum + 1
and t1.ACTIVITYSCORE = t2.ACTIVITYSCORE
and t1.PRODUCT = t2.PRODUCT
and not(t1.GROUP_QUARTILE = t2.GROUP_QUARTILE))
Can anyone help with what should be the t-sql statement for the above?
Cheers!

Answer 1

Assuming you've already worked out a basis Group_Quartile as indicated above, you can update the table with a query similar to the following:
update a
set Group_Quartile = coalesce(topq.Group_Quartile, a.Group_Quartile)
from activityScores a
outer apply
(
select top 1 Group_Quartile
from activityScores topq
where a.Product = topq.Product
and a.Activity_Score = topq.Activity_Score
order by Group_Quartile
) topq
SQL Fiddle with demo.
Edit after comment:
I think you did a lot of the work already by getting the Group_Quartile working.
For each row in the table, the statement above will join another row to it using the outer apply statement. Only one row will be joined back to the original table due to the top 1 clause.
So each for each row, we are returning one more row. The extra row will be matched on Product and Activity_Score, and will be the row with the lowest Group_Quartile (order by Group_Quartile). Finally, we update the original row with this lowest Group_Quartile value so each row with the same Product and Activity_Score will now have the same, lowest possible Group_Quartile.
So SJ003, MH002, etc will all be matched to CH001 and be updated with the Group_Quartile value of CH001, i.e. 2.
It's hard to explain code! Another thing that might help is looking at the join without the update statement:
select a.*
, TopCustomer_id = topq.Customer_Id
, NewGroup_Quartile = topq.Group_Quartile
from activityScores a
outer apply
(
select top 1 *
from activityScores topq
where a.Product = topq.Product
and a.Activity_Score = topq.Activity_Score
order by Group_Quartile
) topq
SQL Fiddle without update.