How to Efficiently Retrieve Values Associated with Maximum Column Values Within Groups in Oracle SQL?

Oracle SQL: Extracting Values Linked to Maximum Column Values within Groups

A common database task involves selecting values associated with the maximum value of a column, especially when dealing with grouped data. Let's consider a table with columns KEY, NUM, and VAL:

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 objective is to find the maximum NUM for each KEY and its corresponding VAL. The desired output:

KEY	VAL
A	AB
B	EF
C	HI
D	LM

One method uses a subquery:

<code class="language-sql">select KEY, VAL
from TABLE_NAME TN
where NUM = (
    select max(NUM)
    from TABLE_NAME TMP
    where TMP.KEY = TN.KEY
    );</code>

However, a more efficient and readable approach employs the ROW_NUMBER() function:

<code class="language-sql">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;</code>

This assigns a rank within each KEY group based on descending NUM values. The WHERE clause then filters for the top rank (seqnum = 1), yielding the desired result:

KEY	VAL
A	AB
B	EF
C	HI
D	LM

While both methods achieve the same outcome, the ROW_NUMBER() approach generally provides better performance and clarity, particularly with larger datasets. The optimal choice depends on specific needs and preferences, but ROW_NUMBER() offers a robust and elegant solution for this common SQL problem.

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