Oracle SQL Query to Extract Last Entry Date per Category

The provided Stack Overflow question seeks an efficient way to extract the most recent records by date per category from a table named events in an Oracle database. The query should return only the most recent records for each distinct value of the category column, along with their corresponding dates.

Background Information

Before diving into the solution, it’s essential to understand the basics of Oracle SQL and its features. In particular, this question relies on several advanced concepts, including:

Date literals and date functions
Windowing functions (specifically, row_number())
Partitioning and ordering in subqueries

The Problem Statement

The original query provided by the user fetches data for a specific customer ID ('860730') within a specified time frame (07-NOV-20 00:00:00 to 08-NOV-20 00:00:00). However, instead of simply retrieving all records within this timeframe, the user wants to extract only the most recent records by date per category. The expected output would be a list of customer IDs paired with their corresponding most recent record dates and categories.

Solution Overview

The suggested solution employs two distinct approaches:

Using row_number() with partitioning and ordering
Aggregation using MAX()

Approach 1: Using `row_number()`

This approach involves adding a new column, seqnum, to the subquery, which assigns a unique number to each record within each category based on its date in descending order. Finally, it selects only the records with a sequence number of 1, effectively retrieving the most recent records per category.

select cust_id, start_time, category 
from (select e.*,
             row_number() over (partition by category order by fme_start_time desc) as seqnum
      from events e
      where cust_id = '860730' and
            fme_start_time >= date '2020-11-07' and
            fme_start_time <  date '2020-11-08'
     ) e
where seqnum = 1;

Approach 2: Aggregation using MAX()

This approach leverages the aggregation capabilities of Oracle SQL to simplify the query. It groups records by customer ID and category, selecting only those with the maximum start time.

select cust_id, max(start_time), category 
from events e
where cust_id = '860730' and
       fme_start_time >= date '2020-11-07' and
       fme_start_time <  date '2020-11-08'
group by cust_id, category;

Comparison of Approaches

While both methods achieve the desired result, they differ in their underlying logic:

row_number() provides a more flexible solution when dealing with multiple columns or additional filtering conditions.
Aggregation using MAX() is simpler and can be more efficient for larger datasets, but it requires that there are no duplicate records with the same maximum start time.

Conclusion

Extracting recent records by date per category in an Oracle database can be achieved through various methods. By understanding the strengths of both windowing functions and aggregation techniques, you can choose the most suitable approach based on your specific requirements and dataset characteristics.

Last modified on 2024-01-08