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

Determining Maximum Signal Values for Unique IDs

The provided table contains multiple signal recordings for the same IDs, making it necessary to retrieve the highest signal value for each ID. Using the MAX() function alone can be problematic as it aggregates all records, affecting the Station and OwnerID columns.

Query Solution with Self-Join and Exclusion

To isolate the highest signal values for each ID, consider a self-join approach. This technique involves joining a table with itself to enable comparisons and filtering.

The following query leverages this approach:

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 operates as follows:

• The cur alias represents the current row being evaluated.
• The nested SELECT statement serves as a subquery to check if a higher signal value exists for the same ID.
• If there is no higher value found, the NOT EXISTS condition returns true, including the current row in the result set.

The output of this query will include a single row for each ID with the highest associated signal value. This approach effectively resolves the issue of aggregating different stations and owner IDs for the same ID.

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