Extracting the Most Recent Entries for Each Group in Oracle SQL

This tutorial demonstrates a common database task: retrieving the most recent record for each group based on a timestamp. We'll use a table containing IDs, timestamps, and quantities as an example.

Challenge:

Given a table with ID, timestamp ("date"), and quantity ("quantity") columns, how do we efficiently select the latest quantity (and its associated timestamp) for every unique ID?

Solution:

This can be achieved using the following approach:

1. Ranking by Timestamp: Assign a rank to each row within each ID group, ordered by timestamp in descending order.
2. Selecting Top Ranks: Filter the results to include only the rows with the highest rank for each ID. These rows represent the latest entries.

The following Oracle SQL query implements this solution:

SELECT x.id, x."date", x.quantity
FROM (
    SELECT
        id,
        "date",
        RANK() OVER (PARTITION BY id ORDER BY "date" DESC) AS rnk,
        quantity
    FROM qtys
) x
WHERE x.rnk = 1;

Extending the Query:

This basic query can be adapted to meet more complex needs:

• Time-Based Filtering: Add a WHERE clause to the inner query to restrict results to a specific time period.
• Joining with Other Tables: Integrate data from additional tables by joining on the id column. For instance, you could join to a table containing ID and name to include names in the output.

This method provides a robust and efficient way to retrieve the latest values per group based on a timestamp in Oracle SQL, applicable to various data analysis and manipulation tasks.