Displaying records with a column value 0 when there is no record

Question

I have following tables:

Schema

tbl_semester_empid {id, semester_name varchar(50), start_date}

tbl_batch_empid {id, semester_id,batch_name, session_room}

tbl_associate_empid {id, associate_name, batch_id, contact, joining_date, induction_result, stream_result int}

The problem query is: Display semester wise associate count.

I have wrote 3 queries:

--basic query
SELECT s.ID "Semester Id", COUNT(*) "Associate Count" FROM TBL_SEMESTER_593932 s, TBL_ASSOCIATE_593932 a, TBL_BATCH_593932 b 
        WHERE s.ID=b.SEMESTER_ID AND a.BATCH_ID=b.ID  
                GROUP BY s.ID;

--used NVL function
SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" FROM TBL_SEMESTER_593932 s, TBL_ASSOCIATE_593932 a, TBL_BATCH_593932 b 
        WHERE s.ID=b.SEMESTER_ID AND a.BATCH_ID=b.ID  
                GROUP BY s.ID;

-- used LEFT OUTER JOIN and  JOIN
SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" 
        FROM TBL_SEMESTER_593932 s LEFT OUTER JOIN TBL_BATCH_593932 b ON s.ID=b.SEMESTER_ID 
             JOIN TBL_ASSOCIATE_593932 a ON a.BATCH_ID=b.ID  
                   GROUP BY s.ID;

All of them display following output (it shows records corresponding to only those semesters which have at least 1 student in it, if theis is no student in that semester, no information for that semester is displayed):

I want the semesters with no associates in them as 0 in Associates Count column. So I tried the same with two LEFT OUTE JOINs:

SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" 
    FROM TBL_SEMESTER_593932 s LEFT OUTER JOIN TBL_BATCH_593932 b ON s.ID=b.SEMESTER_ID 
         LEFT OUTER JOIN TBL_ASSOCIATE_593932 a ON a.BATCH_ID=b.ID  
                GROUP BY s.ID;

However the result is weird:

Answer 1

Basically, what you have to do is to use a list of all semester available and then calculate the amount for each one:

select s.ID, 
       s.Semester_Name,
       NVL ((select count(*) 
          from TBL_ASSOCIATE_593932 a
               join TBL_BATCH_593932 b 
                 on a.BATCH_ID=b.ID
        where  s.ID=b.SEMESTER_ID
       ),0) as Amount
 from TBL_SEMESTER_593932 s

another approach would be left joining your list of all semesters and then count

select s.ID, 
       s.Semester_Name,
       count(*) as Amount 
 from TBL_SEMESTER_593932 s
        left join (
          TBL_BATCH_593932 b
          join TBL_ASSOCIATE_593932 a  
            on a.BATCH_ID=b.ID)
           on s.ID=b.SEMESTER_ID

group by s.ID, s.Semester_Name
order by s.ID

[Edit]: The last query was corrected according to @Mahesha999.

Answer 2

Have you tried using CASE when ?

According to the data in your associate table, there are records for all batches/semesters.

So try this, well your NVL is similar to case...

select s.id, s.name, CASE WHEN count(a.id) IS NULL THEN 0 ELSE count(a.id) END as ACount
from tblsem s
left join tblbatch b
on b.sid = s.id
left join tblaso a
on b.id = a.batchid
group by s.id, s.name
;

Results:

ID  NAME    ACOUNT
1   sem1    4
2   sem2    3
3   sem3    0

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