Compare previous row values for the same id and decide

Question

Hello ,
I have this sample data
create table #student_academic
pk_id int identity(1,1) primary key ,
student_id int ,
[year] int,
[aggr_marks] float
insert into #student_academic
student_id ,
[year] ,
[aggr_marks]
values
(112,2012,55.4),(113,2012,65.4),(114,2012,82.32),
(112,2013,75.4),(113,2013,91.22),(114,2013,45.45),
(112,2014,61.2),(113,2014,95.2),(114,2014,75.6)
select * from #student_academic
from the above data i have to generate an extra status column. this status column should be decided by comparing the previous row value. the exact output should be like below.
student_id year aggr_marks Staus----------------------------------------------------------
112 2012 55.4 None
112 2013 75.4 Promoted
112 2014 61.2 Demoted
113 2012 95.2 None
113 2013 75.6 Demoted
113 2014 91.22 Promoted
113 2012 45.45 None
113 2013 65.4 Promoted
113 2014 82.32 Promoted
is there any way to write the t-sql with out using cursors ? this table has around 25GB of data. Any idea would be appreciated.
Thanks in advance.

Answer 1

Hello,
The difficulty of your example is
that there are several rows for
the same year and the same student.
I present a solution
if there is
one line per year per
student:
TRUNCATE TABLE #student_academicinsert into #student_academic
student_id ,
[year] ,
[aggr_marks]
values
(112,2012,55.4),(113,2012,65.4),(114,2012,82.32),
(112,2013,75.4),(113,2013,91.22),(114,2013,45.45),
(112,2014,61.2),(113,2014,95.2),(114,2014,75.6)
Go
WITH CTE AS
(SELECT pk_id, student_id, year, aggr_marks,
(SELECT TOP 1 aggr_marks FROM #student_academic AS b
WHERE a.student_id = b.student_id AND a.year -1 = b.year
ORDER BY student_id) AS prev_aggr_marks
FROM #student_academic AS a
SELECT
pk_id, student_id, year, aggr_marks,
CASE
WHEN prev_aggr_marks IS NULL THEN 'None'
WHEN aggr_marks < prev_aggr_marks THEN 'Demoted'
WHEN aggr_marks >= prev_aggr_marks THEN 'Promoted'
END AS Status
FROM CTE
ORDER BY student_id, year
Regards,
Charlie
Charlie Dancoisne - Independent Consultant & Trainer (Please take a moment to "Vote as Helpful" and/or "Mark as Answer", where applicable. This helps the community, keeps the forums tidy, and recognises useful contributions. Thanks!)