Get difference between the value in one row and the next - sql

Question

I have some students with report marks over a number of reporting periods. I use the following SQL statement to get those report marks
SELECT DISTINCT ID, thePeriod, MeritRank
FROM
(
SELECT ID,
cast(fileyear as varchar) + cast(filesemester as varchar) as thePeriod,
MeritRank
FROM uNCStudentMeritList) usm
ORDER BY ID, thePeriod asc
This gives me the following data
I would love to have another column which has the difference between each row, partitioned by the ID number. For example
Note: the first value item for each StudentId is left blank as this is the first report mark they have received. From then on, I would like to see the difference between one report mark and the next. If they have received a worse report mark then it should be a negative figure as shown. I don't have row ID numbers or anything in the table - I have seen other similar types of questions answered using row ID numbers. How can I get the sort of results I am after>
Any help would be much appreciated.

Answer 1

Try this :
;WITH cte_getdata
AS (SELECT ID,
Cast(fileyear AS VARCHAR(10))
+ Cast(filesemester AS VARCHAR(10)) AS thePeriod,
MeritRank,
ROW_NUMBER()
OVER (
partition BY id
ORDER BY Cast(fileyear AS VARCHAR(10)) ASC, Cast(filesemester AS VARCHAR(10)) ASC) AS rn
FROM uNCStudentMeritList)
SELECT t1.*,
t1.MeritRank - t2.MeritRank
FROM cte_getdata t1
LEFT JOIN cte_getdata t2
ON t1.rn = t2.rn + 1
AND t1.id = t2.id
The Row_Number() function will assign unique number to each record in partitions of id and order of fileyear,filesemester then you can left join the resultset with itself and match the current row with its previous row having same id. Using Left Join will give you all the rows regardless of matching condition. Here is a same approach with two CTE :
;WITH cte1
AS (SELECT ID,
Cast(fileyear AS VARCHAR)
+ Cast(filesemester AS VARCHAR) AS thePeriod,
MeritRank
FROM uNCStudentMeritList),
cte2
AS (SELECT *,
ROW_NUMBER()
OVER (
partition BY ID
ORDER BY thePeriod) rn
FROM cte1)
SELECT *
FROM cte2 c1
LEFT JOIN cte2 c2
ON c1.id = c2.id
AND c1.rn = c2.rn + 1

Answer 2

Try this.
;WITH cte
AS (SELECT Row_number()OVER (partition BY id ORDER BY theperiod) rn,
*
FROM tablename)
SELECT a.id,
a.theperiod,
a.MeritRank,
a.MeritRank - b.MeritRank
FROM cte A
LEFT JOIN cte b
ON a.rn = b.rn + 1
Or if you are using SQL SERVER 2012+ try this.
SELECT id,
theperiod,
MeritRank,
Lag(MeritRank)
OVER (
partition BY id
ORDER BY theperiod) - MeritRank
FROM tablename

Answer 3

This should also work:
SELECT usm1.ID, usm1.thePeriod, usm1.MeritRank, usm2.difference
FROM (SELECT ID, cast(fileyear as varchar) + cast(filesemester as varchar) as thePeriod, MeritRank
FROM uNCStudentMeritList)usm1 LEFT OUTER JOIN
(SELECT usm3.ID as ID, usm3.thePeriod as thePeriod, max(usm4.thePeriod), (usm3.MeritRank - usm4.MeritRank) as difference
FROM (SELECT ID, cast(fileyear as varchar) + cast(filesemester as varchar) as thePeriod, MeritRank
FROM uNCStudentMeritList)usm3 JOIN
(SELECT ID, cast(fileyear as varchar) + cast(filesemester as varchar) as thePeriod, MeritRank
FROM uNCStudentMeritList)usm4 USING (ID)
WHERE usm3.thePeriod > usm4.thePeriod
GROUP BY usm3.ID, usm3.thePeriod)usm2 USING (ID, thePeriod)