Grouping Rows Based on Column Sum Value

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In this article, we will explore a SQL problem where rows need to be grouped based on the sum of their values. The goal is to ensure that no group has a sum greater than a specified limit.

Problem Statement

Given a table with three columns: id, num_rows, and an unknown third column, we want to group the rows such that the sum of num_rows for each group is never above a specified value (in this case, 500). If an id has a value greater than the limit, it gets its own separate group.

Background

The problem at hand is closely related to the Knapsack Problem, which is a classic problem in computer science and operations research. The Knapsack Problem involves finding the optimal way to pack items of different weights into a knapsack of limited capacity.

In our case, we can treat each group as an “item” and the sum of num_rows for each group as the weight. However, unlike the traditional Knapsack Problem, where the goal is to maximize the total value while staying within the capacity limit, our goal is to minimize the maximum sum across all groups.

SQL Solution

There are several ways to approach this problem in SQL. In this section, we will explore two solutions: one using a PL/SQL function and another using pure SQL.

Pl/SQL Function Solution

A common approach to solve this problem is by creating a PL/SQL function that can handle the grouping logic. Here’s an example implementation:

CREATE OR REPLACE FUNCTION group_rows_by_sum()
  RETURN SYS_REFCURSOR AS
BEGIN
  OPEN :rec FOR
  SELECT id, num_rows,
         SUM(CASE WHEN num_rows > 500 THEN 1 ELSE 0 END) OVER (ORDER BY num_rows) AS group_id
  FROM my_table;
  RETURN;
END;
/;

BEGIN
  FOR rec IN AGGREgate group_rows_by_sum() LOOP
    DBMS_OUTPUT.PUT_LINE(rec);
  END LOOP;
END;
/

However, this solution has a limitation. It only works for the specific column sum limit specified in the SUM function.

Pure SQL Solution

For those who prefer a pure SQL solution, we can use Common Table Expressions (CTEs) to solve the problem:

WITH ord AS (
  SELECT id, num_rows, ROWNUM ord FROM my_table
)
, rek(ord, id, num_rows, sum_rows, groupId) AS (
  SELECT ord, id, num_rows, num_rows, 1 FROM ord WHERE ord = 1
  UNION ALL
  SELECT rek.ord +1, ord.id, ord.num_rows,
         CASE WHEN rek.sum_rows + ord.num_rows > 500 THEN ord.num_rows ELSE rek.sum_rows + ord.num_rows END AS sum_rows,
         CASE WHEN rek.sum_rows + ord.num_rows > 500 THEN rek.groupID + 1 ELSE rek.groupID END AS groupId
    FROM rek
    JOIN ORD ON ord.ord = rek.ord+1
)
SELECT id, num_rows, groupId
FROM rek;

This solution uses a recursive CTE to build the groups based on the sum of num_rows. The recursion stops when the sum exceeds 500, and the current row is added to the previous group.

How it Works

Let’s break down how the SQL solution works:

1. We create a temporary table ord with the original data from the my_table.
2. We define a recursive CTE rek that takes the current row number as input.
3. The first part of the union selects the first row and assigns it to the initial group ID.
4. The second part of the union recursively joins the previous group’s rows with the current row, adding the sum of their values and updating the group ID accordingly.
5. When the recursion stops (i.e., when the sum exceeds 500), the last row is assigned to a new group.

Conclusion

In this article, we explored how to group rows based on the sum of their values in SQL. We presented two solutions: one using a PL/SQL function and another using pure SQL with Common Table Expressions.

While both solutions have their own limitations, they demonstrate that grouping rows by sum is a fundamental problem in data analysis that can be solved using creative SQL techniques.

Last modified on 2024-10-06