Understanding Row Numbering and Sub Grouping in Oracle SQL

In this article, we will explore the concept of row numbering and sub-grouping in Oracle SQL. We will examine how to use the ROW_NUMBER and DENSE_RANK analytic functions to achieve the desired output.

Background

Row numbering is a technique used to assign a unique number to each row in a result set based on a specific criteria, such as an ordering column or a group identifier. In the context of SQL, row numbering can be achieved using various windowing functions, including ROW_NUMBER, RANK, and DENSE_RANK.

Sub-grouping is another important concept in data analysis that involves grouping rows based on certain conditions, such as a specific column value. Sub-grouping can help to identify patterns or trends within the data.

Problem Statement

The problem at hand is to create a list of activities ordered by activity ID and time with an incremental ID for each activity. Additionally, we want to include a secondary column that starts from 1 and increments when the status differs from the previous row.

Given the following example dataset:

We want to achieve the following output:

Solution

To achieve the desired output, we can use a combination of windowing functions and grouping.

First, let’s use ROW_NUMBER to assign an incremental ID to each activity ordered by event timestamp:

SELECT t.*, 
       ROW_NUMBER() OVER (PARTITION BY ACTIVITY_ID ORDER BY EVENT_TIMESTAMP) 
                                                                         AS EVENT_NUMBER,
       DENSE_RANK() OVER (PARTITION BY ACTIVITY_ID ORDER BY EVENT_STATUS ) 
                                                                   AS EVENT_STATUS_GROUP
  FROM tab t
ORDER BY ACTIVITY_ID, EVENT_NUMBER;

This will give us the following output:

However, this output does not meet our requirement of having an incremental ID for each status change.

To achieve this, we can use DENSE_RANK instead of ROW_NUMBER. The main difference between the two is that ROW_NUMBER assigns a unique number to each row within each partition (i.e., group), whereas DENSE_RANK assigns a rank that is the same for consecutive ranks.

Here’s how you can modify the query:

SELECT t.*, 
       DENSE_RANK() OVER (PARTITION BY ACTIVITY_ID ORDER BY EVENT_STATUS) AS EVENT_NUMBER,
       DENSE_RANK() OVER (PARTITION BY ACTIVITY_ID, EVENT_STATUS ORDER BY 1) AS EVENT_STATUS_GROUP
  FROM tab t
ORDER BY ACTIVITY_ID, EVENT_NUMBER;

This will give us the desired output:

By using DENSE_RANK, we get an incremental ID for each status change, as required.

Conclusion

In conclusion, row numbering and sub-grouping are essential techniques in data analysis. By leveraging windowing functions like ROW_NUMBER and DENSE_RANK, you can create elegant solutions to complex problems. In this article, we explored how to use these functions to achieve an incremental ID for each activity with a secondary column that increments when the status changes.

I hope this explanation helps! Let me know if you have any further questions or need more clarification on any of the concepts discussed here.

Last modified on 2024-05-01