Oracle count distinct record within subquery - oracle

Question

I have 3 tables
SUBJECTS
CODE, SUBJECT_NAME , SESSION
100, MATHS , AM
101, MATHS - INTRO , AM
102, MATHS - ADVANCED , AM
200, ENGLISH , AM
201, ENGLISH - INTRO , AM
202, ENGLISH - BEGINNER, AM
203, ENGLISH - ADVANCED, AM
STUDENTS_SUBJECTS
ID, SUBJECT_CODE
2, 101
2, 102
1, 201
1, 203
3, 101
3, 102
STUDENTS
ID,PARENT_ID, STUDENT_NAME, CLASS_LEADER, INACTIVE, EXPERT
1 , 2 , ELSA , no , N , N
2 , 4 , STEVE , no , N , N
3 , 5 , MIKE , no , N , N
My query goes like
SELECT t1.CODE,
t1.SUBJECT_NAME,
SUM (CASE WHEN ( (t2.CLASS_LEADER = 'no'
OR t2.CLASS_LEADER IS NULL)
AND t2.EXPERT IS NULL)
THEN 1 ELSE 0 END) AS "Average Student"
FROM subjects t1
LEFT OUTER JOIN (
select a.STUDENT_ID, a.PARENT_ID, a.STUDENT_NAME,
a.CLASS_LEADER, c.SUBJECT_CODE, a.INACTIVE, a.EXPERT
FROM students a
INNER JOIN students_subjects c
ON (a.STUDENT_ID = c.ID )
where (INACTIVE is null)
GROUP BY a.STUDENT_ID, a.PARENT_ID, a.STUDENT_NAME, a.CLASS_LEADER, c.SUBJECT_CODE, a.INACTIVE, a.EXPERT
) t2
ON substr(trim(t2.SUBJECT_CODE),1,2)= substr(trim(t1.CODE),1,2)
WHERE (t1.SESSION='AM')
GROUP BY t1.CODE, T1.SUBJECT_NAME
ORDER BY T1.CODE
What I would like to get is the number of students who signed up for the class for morning session under each major subject without the duplicates. For example, each students who signed up for Maths - Intro & Maths Advanced should only be counted once under the Maths subject.
if I run the subquery separately minus the subject_code in select statement and group by statement, I managed to get the correct value however I'm not sure how to return the correct value when it's joined in the query.
REPORT
CODE, SUBJECT_NAME, AVERAGE_STUDENT
100 MATHS 2
200 ENGLISH 1
Thank you.

Answer 1

First some recomendation:
1) add column MAIN_SUBJECT_CODE to the table SUBJECTS (as already commented)
2) the column ID in the table STUDENTS_SUBJECTS is a foreign key pointing to the table STUDENT, so a better name will be STUDENT_ID
3) use unique mechanism to store Boolean values do not mix 'no' and 'N'
First the query of all student subscriptions
Note that I added the missing column main_subject_code and adjusted the average student definition to get some result.
SELECT su.CODE,
substr(trim(su.CODE),1,2)||'0' main_subject_code,
su.SUBJECT_NAME,
st.STUDENT_NAME,
CASE WHEN ( (st.CLASS_LEADER = 'no'
OR st.CLASS_LEADER IS NULL)
AND st.EXPERT = 'N' /*IS NULL*/)
THEN 1 ELSE 0 END AS "Average Student"
FROM subjects su
INNER JOIN students_subjects ss
ON su.code = ss.SUBJECT_CODE
INNER JOIN STUDENTS st
ON ss.ID /* STUDENT_ID */ = st.ID
;
CODE MAIN_SUBJECT_CODE SUBJECT_NAME STUDENT_NAME Average Student
101 100 MATHS - INTRO MIKE 1
101 100 MATHS - INTRO STEVE 1
102 100 MATHS - ADVANCED MIKE 1
102 100 MATHS - ADVANCED STEVE 1
201 200 ENGLISH - INTRO ELSA 1
203 200 ENGLISH - ADVANCED ELSA 1
The rest is simple - group on main subject and add the title of it
with subsr as (
SELECT su.CODE,
substr(trim(su.CODE),1,2)||'0' main_subject_code,
su.SUBJECT_NAME,
st.STUDENT_NAME,
CASE WHEN ( (st.CLASS_LEADER = 'no'
OR st.CLASS_LEADER IS NULL)
AND st.EXPERT = 'N' /*IS NULL*/)
THEN 1 ELSE 0 END AS "Average Student"
FROM subjects su
INNER JOIN students_subjects ss
ON su.code = ss.SUBJECT_CODE
INNER JOIN STUDENTS st
ON ss.ID /* STUDENT_ID */ = st.ID
)
select
main_subject_code,
(select SUBJECT_NAME from SUBJECTS where CODE = main_subject_code) main_subject_name,
sum("Average Student") "Average Student"
from subsr
group by main_subject_code
order by main_subject_code;
MAIN_SUBJECT_CODE MAIN_SUBJECT_NAME Average Student
----------------- ------------------------- ---------------
100 MATHS 4
200 ENGLISH 2

Answer 2

Your posted query contains a lot of extraneous logic which doesn't seem releavnt to your apparent task. So I'm ignoring it and focusing on simply getting "the number of students who signed up for the class for morning session under each major subject without the duplicates".
select major
, count(*)
from (
select distinct subj.major
, ss.id as student_id
from
( select code,
regexp_replace(subject_name, '^([A-Z]+)(.*)', '\1') major ,
from subjects
where session = 'AM'
) subj
join student_subjects ss
on ss.subject_code = subj.code
)
group by major
order by major
/
The subquery on SUBJECTS use a regex function to extract the leading element of the subject name as the major. It works for the posted sample data but might fail for more complicated names. Regex shouldn't be necessary: a proper data model would separate the MAJOR subject from its subsidiaries.