How Can I Efficiently Select Maximum-Based Values in Oracle SQL Using Window Functions?

Oracle SQL: Extracting Maximum Values with Window Functions

Efficiently retrieving specific column values based on the maximum value of another column, while grouping by a third, is easily accomplished in Oracle SQL using window functions. This approach offers elegance and performance advantages over alternative methods.

Consider this sample table:

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 Window Function Solution

The optimal solution leverages the ROW_NUMBER() window 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 query yields the following result:

KEY	VAL
A	AB
B	EF
C	HI
D	LM

How it Works

1. The OVER (PARTITION BY key ORDER BY num DESC) clause partitions the data by the KEY column and orders each partition by the NUM column in descending order.
2. ROW_NUMBER() assigns a unique rank to each row within its partition. The row with the highest NUM value receives a rank of 1.
3. The outer WHERE clause filters the results, retaining only rows with seqnum = 1, effectively selecting the maximum NUM value for each KEY.

Key Difference

A crucial point: Unlike alternative methods, the window function approach guarantees a single row per KEY, even if multiple rows share the same maximum NUM value. This behavior might differ from other techniques that could return multiple rows in such cases. If this distinction is unimportant, the window function offers a superior solution due to its clarity and efficiency.

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