Unpivot Columns with Most Recent Record - tsql

Question

Student Records are updated for subject and update date. Student can be enrolled in one or multiple subjects. I would like to get each student record with most subject update date and status.
CREATE TABLE Student
(
StudentID int,
FirstName varchar(100),
LastName varchar(100),
FullAddress varchar(100),
CityState varchar(100),
MathStatus varchar(100),
MUpdateDate datetime2,
ScienceStatus varchar(100),
SUpdateDate datetime2,
EnglishStatus varchar(100),
EUpdateDate datetime2
);
Desired query output, I am using CTE method but trying to find alternative and better way.
SELECT StudentID, FirstName, LastName, FullAddress, CityState, [SubjectStatus], UpdateDate
FROM Student
;WITH orginal AS
(SELECT * FROM Student)
,Math as
(
SELECT DISTINCT StudentID, FirstName, LastName, FullAddress, CityState,
ROW_NUMBER OVER (PARTITION BY StudentID, MathStatus ORDER BY MUpdateDate DESC) as rn
, _o.MathStatus as SubjectStatus, _o.MupdateDate as UpdateDate
FROM original as o
left join orignal as _o on o.StudentID = _o.StudentID
where _o.MathStatus is not null and _o.MUpdateDate is not null
)
,Science AS
(
...--Same as Math
)
,English AS
(
...--Same As Math
)
SELECT * FROM Math WHERE rn = 1
UNION
SELECT * FROM Science WHERE rn = 1
UNION
SELECT * FROM English WHERE rn = 1

Answer 1

First: storing data in a denormalized form is not recommended. Some data model redesign might be in order. There are multiple resources about data normalization available on the web, like this one.
Now then, I made some guesses about how your source table is populated based on the query you wrote. I generated some sample data that could show how the source data is created. Besides that I also reduced the number of columns to reduce my typing efforts. The general approach should still be valid.
Sample data
create table Student
(
StudentId int,
StudentName varchar(15),
MathStat varchar(5),
MathDate date,
ScienceStat varchar(5),
ScienceDate date
);
insert into Student (StudentID, StudentName, MathStat, MathDate, ScienceStat, ScienceDate) values
(1, 'John Smith', 'A', '2020-01-01', 'B', '2020-05-01'),
(1, 'John Smith', 'A', '2020-01-01', 'B+', '2020-06-01'), -- B for Science was updated to B+ month later
(2, 'Peter Parker', 'F', '2020-01-01', 'A', '2020-05-01'),
(2, 'Peter Parker', 'A+', '2020-03-01', 'A', '2020-05-01'), -- Spider-Man would never fail Math, fixed...
(3, 'Tom Holland', null, null, 'A', '2020-05-01'),
(3, 'Tom Holland', 'A-', '2020-07-01', 'A', '2020-05-01'); -- Tom was sick for Math, but got a second chance
Solution
Your question title already contains the word unpivot. That word actually exists in T-SQL as a keyword. You can learn about the unpivot keyword in the documentation. Your own solution already contains common table expression, these constructions should look familiar.
Steps:
cte_unpivot = unpivot all rows, create a Subject column and place the corresponding values (SubjectStat, Date) next to it with a case expression.
cte_recent = number the rows to find the most recent row per student and subject.
Select only those most recent rows.
This gives:
with cte_unpivot as
(
select up.StudentId,
up.StudentName,
case up.[Subject]
when 'MathStat' then 'Math'
when 'ScienceStat' then 'Science'
end as [Subject],
up.SubjectStat,
case up.[Subject]
when 'MathStat' then up.MathDate
when 'ScienceStat' then up.ScienceDate
end as [Date]
from Student s
unpivot ([SubjectStat] for [Subject] in ([MathStat], [ScienceStat])) up
),
cte_recent as
(
select cu.StudentId, cu.StudentName, cu.[Subject], cu.SubjectStat, cu.[Date],
row_number() over (partition by cu.StudentId, cu.[Subject] order by cu.[Date] desc) as [RowNum]
from cte_unpivot cu
)
select cr.StudentId, cr.StudentName, cr.[Subject], cr.SubjectStat, cr.[Date]
from cte_recent cr
where cr.RowNum = 1;
Result
StudentId StudentName Subject SubjectStat Date
----------- --------------- ------- ----------- ----------
1 John Smith Math A 2020-01-01
1 John Smith Science B+ 2020-06-01
2 Peter Parker Math A+ 2020-03-01
2 Peter Parker Science A 2020-05-01
3 Tom Holland Math A- 2020-07-01
3 Tom Holland Science A 2020-05-01