SQL same postal code - sql

Question

select PERSON.fornavn, PERSON.efternavn, PERSON.postnr, POST.distrikt
from PERSON inner join POST on POST.postnr = PERSON.postnr
inner join MEDLEM on PERSON.personnr = MEDLEM.personnr
inner join FORMAND on MEDLEM.personnr = FORMAND.personnr
group by PERSON.fornavn, PERSON.efternavn, PERSON.postnr, POST.distrikt, FORMAND.afdnr
having FORMAND.afdnr = 3
order by PERSON.fornavn
I need to show the persons that live on the same postal code as the boss for department 3, but the results make no sense.
Fornavn = First name, Formand = boss, Adresse = adress, Afdeling = Department (afdnr = department nr.), Medlem = member
Tables:
PERSON: personnr, fornavn, adresse, postnr
POST: postnr, district
FORMAND: personnr, afdnr
MEDLEM: personnr, afdnr
AFDELING: afdnr, afdname

Answer 1

select PERSON.fornavn, PERSON.efternavn, PERSON.postnr, POST.distrikt
from PERSON inner join POST on POST.postnr = PERSON.postnr
inner join MEDLEM on PERSON.personnr = MEDLEM.personnr
inner join FORMAND on MEDLEM.personnr = FORMAND.personnr
where FORMAND.afdnr = 3
group by PERSON.fornavn, PERSON.efternavn, PERSON.postnr, POST.distrikt, FORMAND.afdnr
order by PERSON.fornavn
try with :
where FORMAND.afdnr = 3
insteed of
having FORMAND.afdnr = 3

Answer 2

If I understand you right ("the persons that live on the same postal code as (the boss for department 3)", and assuming there is only one boss:
select PERSON.fornavn, PERSON.efternavn, PERSON.postnr, POST.distrikt
from PERSON
inner join POST on POST.postnr = PERSON.postnr
where PERSON.postnr = (SELECT pf.postnr
FROM FORMAND f
inner join PERSON pf ON f.personnr = pf.personnr
WHERE f.afdnr = 3)
order by PERSON.fornavn
Note: Your query makes no sense given the description, unless I am missing something. You are joining PERSON through MEDLEM to FORMAND on personnr, which means you will only get the boss's record back (PERSON.personnr = FORMAND.personnr).
Note: If there is more than one boss, you can change the = in the WHERE clause to IN to get all people that live in the same post code as any of the bosses.
In case I read you wrong, and you actually mean "the (persons that live on the same postal code as the boss) for department 3", try:
select p.fornavn, p.efternavn, p.postnr, POST.distrikt
from PERSON p
inner join POST on POST.postnr = p.postnr
inner join MEDLEM on p.personnr = MEDLEM.personnr
inner join FORMAND on MEDLEM.afdnr = FORMAND.afdnr
inner join PERSON pf on FORMAND.personnr = pf.personnr AND pf.postnr = p.postnr
where MEDLEM.afdnr = 3
order by PERSON.fornavn
Note: Again, I believe your join from the question is strange. See above.