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 only the first seenTime

Records:

seenID | personID | seenTime
   108      3         13:34
   109      2         13:56
   110      3         14:22
   111      3         14:31
   112      4         15:04
   113      2         15:52

Wanted result:

personID | seenTime
   3         13:34
   2         13:56
   4         15:04

That's what I did & failed:

SELECT t.attendanceID, t.seenPersonID, t.seenTime
(SELECT ROW_NUMBER() OVER (PARTITION BY seenID ORDER BY seenID) AS RowNo,
seenID,
seenPersonID,
seenTime
FROM personAttendances) t
WHERE t.RowNo=1

PS: Notice SQL CE 4

Answer 1

If your seenTime increases as seenID increases:

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

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

You're making it way too difficult:

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

Answer 3

对于PostgreSQL，有DISTINCT ON

Answer 4

您需要按看到的时间而不是看到的 id 订购：

PARTITION BY seenID ORDER BY seenTime

Answer 5

Add this to your SQL:

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