SELECT To obtain results from two tables where a value occurs once only in a table column - sql

Question

Sorry I know this should be relatively simple and probably has been answered before, but I've got myself blocked on this and can't find a post here that matches.
I have two tables companies and contacts. A contact can be linked to 0 or 1 company, a company can have 1 or more contacts and MAY have one set as a 'primary contact'
companies(ccyID,ccyname,primconID)
________________________________________________
| ccyID | ccyname | PrimconID |
------------------------------------------------
| aaaaaaa | Company A | NULL |
| bbbbbbb | Company B | NULL |
| ccccccc | Company C | vvvvvvv |
________________________________________________
contacts(conID,firstname,lastname,ccyID)
__________________________________________________
| conID | first | last | companyID |
--------------------------------------------------
| zzzzzzz | Stand | Alone | NULL |
| yyyyyyy | Only | Contact | aaaaaaa |
| xxxxxxx | CompanyB | First | bbbbbbb |
| wwwwwww | CompanyB | Second | bbbbbbb |
| vvvvvvv | CompanyC | Only | ccccccc |
_________________________________________________
I need a SELECT that will return the companyID and contactID when the company has exactly one contact, AND does not have a PrimconID set i.e. for the above data I want returned
conID ccyID
----------------
yyyyyyy aaaaaaa
(The eventual idea is that I'm then going to update the tables to make the solitary contact for companies the primary contact)

Answer 1

Group contacts by company to keep only those with a count of 1 and make sure they are in the set of companies without primary contact:
select companyid, max(conid)
from contacts
group by companyid
having count(*) = 1
and companyid in (select ccyid from companies where primconid is null)
order by companyid;

Answer 2

I would use a join and group by:
select c.ccyid, max(co.conid)
from companies c join
contacts co
on co.companyid = c.ccyid
where c.PrimconID is null
group by c.ccyid
having min(co.conID) = max(co.conID);
As an update statement, you can do:
update c
set PrimconID = co.conid
from companies c join
(select co.companyid, max(co.conid) as conid
from contacts co
group by companyid
having min(co.conid) = max(co.conid)
) co
on co.companyid = c.ccyid
where c.PrimconID is null ;

Answer 3

Use a CTE to first identify all contacts having only a single company, and then join to the company table:
WITH cte AS (
SELECT ccyID, MAX(conID) AS conID
FROM contacts
GROUP BY ccyID
HAVING COUNT(*) = 1
)
SELECT
t.conID,
c.ccyID
FROM companies c
INNER JOIN cte t
ON c.ccyID = t.ccyID
WHERE
c.PrimconID IS NULL;
See the demo below to see the query working.
Demo