Query for Joining three tables with row value as column header - sql

Question

I have three tables as shown below.
TABLE1 : tb_subject
subject_id subject_name
1 English
2 Maths
3 Science
Table2 : tb_student
subject_id student_id
1 AA
1 BB
2 CC
3 DD
3 EE
Table3 : tb_student_score
student_id score conducted_month_number
AA 20 2
BB 30 3
CC 50 4
AA 80 4
DD 50 6
BB 10 2
EE 40 3
Result should be
conducted_month_number SUM(subject_id1) SUM(subject_id2) SUM(subject_id3)
1 0 0 0
2 30 0 0
3 30 0 40
4 80 50 0
5 0 0 0
6 0 0 60
7 0 0 0
8 0 0 0
9 0 0 0
10 0 0 0
11 0 0 0
12 0 0 0
How to write a select query for this? Can add all month number that is not stored in table as like in the resulted output?

Answer 1

You should be able to use case when to sum for each subject individually:
SELECT conducted_month_number,
SUM(CASE b.subject_id WHEN 1 THEN a.score ELSE 0 END) AS English,
SUM(CASE b.subject_id WHEN 2 THEN a.score ELSE 0 END) AS Maths,
SUM(CASE b.subject_id WHEN 3 THEN a.score ELSE 0 END) AS Science
FROM tb_student_score AS a
JOIN tb_student AS b ON b.student_id = a.student_id
GROUP BY conducted_month_number
ORDER BY conducted_month_number;
However, this alone will not ensure you have results for values of conducted_month_number that don't exist - if this is an issue, you could simply create a dummy student with a score of 0 for each month.
Edit: I noticed some comments posted around the same time I submitted my answer - if you want the number of summation columns to be variable based on the values of rows in the tb_subject table, you will not find the relational model of SQL to be well suited for that task. However, you can easily go back and update your query to include any new subjects you may add later on.

Answer 2

Have added dummy values of 1 to 12 months using union statement and later on did group by on them to calculate total scores.
Try this:-
Select conducted_month_number ,
sum(case when subject_id=1 then score else 0 end) as sum_subject_id1,
sum(case when subject_id=2 then score else 0 end) as sum_subject_id2,
sum(case when subject_id=3 then score else 0 end) as sum_subject_id3
from
(
Select a.conducted_month_number ,subject_id,score
from
tb_student_score a
inner join
tb_student b
on a.student_id=b.student_id
union
select 1,' ',0 from tb_student_score
union
select 2,' ',0 from tb_student_score
union
select 3,' ',0 from tb_student_score
union
select 4,' ',0 from tb_student_score
union
select 5,' ',0 from tb_student_score
union
select 6,' ',0 from tb_student_score
union
select 7,' ',0 from tb_student_score
union
select 8,' ',0 from tb_student_score
union
select 9,' ',0 from tb_student_score
union
select 10,' ',0 from tb_student_score
union
select 11,' ',0 from tb_student_score
union
select 12,' ',0 from tb_student_score
)a
group by conducted_month_number
My Output
conducted_month_number sum_subject_id1 sum_subject_id2 sum_subject_id3
1 0 0 0
2 30 0 0
3 30 0 40
4 80 50 0
5 0 0 0
6 0 0 50
7 0 0 0
8 0 0 0
9 0 0 0
10 0 0 0
11 0 0 0
12 0 0 0