How to Find the Maximum Signal Value for Each Unique ID in a Dataset?

Retrieving Maximum Signal Values for Unique IDs

In order to determine the maximum signal value for each unique ID in a given dataset, you can employ the following approach, which involves a self-join operation:

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 query works by joining the table with itself (represented by the aliases cur and high) and filtering out rows for which a higher signal value exists. The result is a list of rows that contain the ID, signal value, station, and ownerID for each distinct ID with the highest signal value.

In the context of the example table provided:

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

Executing the query would yield the following results:

ID    Signal    Station    OwnerID
111   -120      Home        1
222   -95       Work        1

This table contains the maximum signal value for each unique ID, effectively solving the original problem without unnecessary data aggregation or the need for additional table joins.

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