Question

I need to select rows that have one treated row for one country and there exists one untreated row for the same country. For below table I would then return row with id = 1.

So first I need to check if there is a row with TREATED status, then I have to check if there is a row with UNTREATED status and the same country as the one with the TREATED status.

    Table example 1
    +----+--------+--------------+
    | id | country| status       |
    +----+--------+--------------+
    | 1  | SE     | TREATED      |
    | 2  | DK     | UNTREATED    |
    | 3  | SE     | UNTREATED    |
    +----+--------+--------------+

    Result of query
    +----+--------+--------------+
    | id | country| status       |
    +----+--------+--------------+
    | 1  | SE     | TREATED      |
    +----+--------+--------------+

If there does not exist an untreated row or the country is not the same then we should not return anything

    Table example 2
    +----+--------+--------------+
    | id | country| status       |
    +----+--------+--------------+
    | 1  | SE     | TREATED      |
    | 2  | DK     | UNTREATED    |
    | 3  | US     | UNTREATED    |
    +----+--------+--------------+

    Result of query (empty)
    +----+--------+--------------+
    | id | country| status       |
    +----+--------+--------------+

Answer 1

Here are a couple of other options:

select t1.id,t1.country,t1.status
    from table t1
    join table t2 on t1.country=t2.country and t2.status='UNTREATED'
    where t1.status='TREATED'


 select id,country,status
            from table t1
            where 
                 t1.status='TREATED' and
                 exists (
                      select 1 from table t2 where 
                           t2.country = t1.country and 
                           t2.status = 'UNTREATED'
                 )

I believe the first one, using a join, would probably perform the best.