SQL: Return only first occurrence

Question

I seldomly use SQL and I cannot find anything similar in my archive so I\'m asking this simple query question: I need a query which one returns personID and onl

Answer 1

If your seenTime increases as seenID increases:

select personID, min(seenTime) as seenTime
from personAttendances
group by personID

Update for another case:

If this is not the case, and you really want the seenTime that corresponds with the minimum seenID (assuming seenID is unique):

select a.personID, a.seenTime
from personAttendances as a
    join (
        -- Get the min seenID for each personID
        select personID, min(seenID) as seenID
        from personAttendances
        group by personID
    ) as b on a.personID = b.personID
where a.seenID = b.seenID

Answer 2

Add this to your SQL:

and where not exists
    (select 1 from personAttendances t2 
    where t.personID=t2.personID 
    and t2.seenID < t.seenID)

Answer 3

You're making it way too difficult:

select personID, min(seenTime)
from personAttendances
group by personID

Answer 4

You need to order by seen time not by seen id:

PARTITION BY seenID ORDER BY seenTime

Answer 5

for PostgreSQL there is DISTINCT ON