Simple Query to Grab Max Value for each ID

greatest-n-per-group, max, sql, sql-server

Solution

Something like this? Join your table with itself, and exclude the rows for which a higher signal was found.

select cur.id, cur.signal, cur.station, cur.ownerid
from yourtable cur
where not exists (
    select * 
    from yourtable high 
    where high.id = cur.id 
    and high.signal > cur.signal
)

This would list one row for each highest signal, so there might be multiple rows per id.

Problem

OK I have a table like this: ``` ID Signal Station OwnerID 111 -120 Home 1 111 -130 Car 1 111 -135 Work 2 222 -98 Home 2 222 -95 Work 1 222 -103 Work 2 ``` This is all for the same day. I just need the Query to return the max signal for each ID: ``` ID Signal Station OwnerID 111 -120 Home 1 222 -95 Work 1 ``` I tried using MAX() and the aggregation messes up with the Station and OwnerID being different for each record. Do I need to do a JOIN?

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