How Can I Efficiently Retrieve Values Based on the Maximum of a Related Column in SQL?

Efficiently obtain data based on the maximum value of the relevant column

In scenarios where you need to retrieve a specific column value based on the maximum value of the related column and grouped by a third column, SQL provides an optimized solution. Consider the following table, which contains data for KEY, NUM, and VAL columns:

KEY	NUM	VAL
A	1	AB
B	1	CD
B	2	EF
C	2	GH
C	3	HI
D	1	JK
D	3	LM

The goal is to retrieve the following results from this data:

KEY	VAL
A	AB
B	EF
C	HI
D	LM

While the query provided in the question achieves this result, there is a more elegant way using row_number():

select key, val
from (
  select t.*, row_number() over (partition by key order by num desc) as seqnum
  from table_name t
) t
where seqnum = 1;

This query efficiently partitions rows based on the KEY column and assigns row numbers in descending order seqnum within each partition. By selecting rows where seqnum is equal to 1, we obtain the desired result and retain the desired grouping.

It's worth noting that this method differs slightly from the original query in that it always returns one row per key, whereas the original query may return multiple rows. If this behavior is desired, rank() or dense_rank() can be used instead of row_number().

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