Having clause question maybe

All we need is an easy explanation of the problem, so here it is.

I have 3 tables.

  1. Grades
  2. Disciplines
  3. Students

I want to select only the disciplines that doesn’t have grade 2. What am I doing wrong?

SELECT DISTINCT DISC_NAME
FROM STUDENTS, DISCIPLINES, GRADES
WHERE 
GRADES.ID= STUDENTS.ID
AND
GRADES.KOD_DISC = DISCIPLINES.DISC_ID
GROUP BY GRADE
HAVING
GRADE != 2

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

HAVING is for filtering on aggregated results. You could use it to solve your problem if you rewrite it slightly so it’s filtering on the number of times a DISC_NAME has a result from your joins which has GRADE=1. For this aggregate, you’d be grouping by the DISC_NAME and you can count matches with a case when

select d.disc_name
from students s
    ,disciplines d
    ,grades g
where g.id= s.id
and   g.kod_disc = d.disc_id
group by d.disc_name
having sum(case when g.grade = 2 then 1 else 0 end) = 0

(assuming the grade column comes from grades and disc_name is part of disciplines)

You could also achieve the same with an anti-join query. Here I’ve removed the join to students because you don’t seem to use it (other than making sure the grade row belongs to a student row, which I would assume isn’t necessary). I’ve also switched to using more modern ansi join syntax – you could use either.

select d.disc_name
from disciplines d
where not exists (select null
                  from   grades g
                  where  d.disc_id = g.disc_id
                  and    g.grade = 2
                 )

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

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