Using Window Functions to Select the Latest Date for Each ID Video Type

When working with data from different sources, it’s not uncommon to encounter situations where you need to process or analyze data based on specific conditions. In this case, we’re dealing with a database table that stores information about videos, including their type and insertion date. The goal is to select all the last dates from all list of id video_type without repeating any ID_video_type.

Understanding the Problem

Let’s take a closer look at the provided example data:

As you can see, there are multiple videos for each id_video_tipo, and we want to select the latest date for each group without repeating any id_video_tipo.

The Current Approach

The original approach uses a simple GROUP BY statement to group data by id_video_tipo and then sorts it in descending order by the data column. However, this method has a limitation: it doesn’t account for the fact that we want to select only the latest date for each group.

SELECT *
FROM `video`
GROUP BY id_video_tipo
ORDER BY data DESC;

This query will return all videos with the same id_videoTipo, but not necessarily the most recent one. For example, for id_videotipo = 6, it shows video with id = 2 instead of id = 3.

The Solution

To achieve our goal, we need to use a WINDOW function that allows us to access data within a partition of the result set. In this case, we’ll utilize the ROW_NUMBER() function over a PARTITION BY clause.

CREATE TABLE video
(
    id_video_tipo INT,
    data DATETIME
);

INSERT INTO video VALUES (1,'2020-01-01'),(1,'2020-01-02'),(1,'2020-01-03'),(1,'2020-01-04');

First, let’s create the table with some sample data.

SELECT *
FROM
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY id_video_tipo ORDER BY data DESC) AS 'RNum'
    FROM `video` 
) vids
WHERE vids.RNum = 1;

Here’s what’s happening in this query:

• We’re creating a subquery that selects all columns from the video table.
• We’re using the ROW_NUMBER() function over a PARTITION BY clause to assign a unique row number within each partition (in this case, by id_video_tipo). The rows are ordered in descending order by the data column.
• We then select only those rows where the RNum is equal to 1. This effectively selects the most recent date for each group.

Example Walkthrough

Let’s walk through an example to illustrate how this works:

Suppose we have the following data in our table:

Here’s what happens when we run the query:

• For id_video_tipo = 1, the rows are ordered by date in descending order. Since there are two rows for this type, one newer than the other (2019-12-31 and 2020-01-01), ROW_NUMBER() assigns the values as follows:
  • First row: RNum = 2 (older date)
  • Second row: RNum = 1 (newer date)

So, we select only rows with RNum = 1, which gives us:

For id_video_tipo = 2, there’s only one row. ROW_NUMBER() assigns the value as follows:

• First (and only) row: RNum = 1

So, we select only that row:

For id_video_tipo = 3, there’s also only one row. ROW_NUMBER() assigns the value as follows:

• First (and only) row: RNum = 1

So, we select only that row:

This results in the following output:

Conclusion

In this article, we explored how to use WINDOW functions in SQL to select the latest date for each group without repeating any ID_video_type. We covered the problem, current approach, and solution using ROW_NUMBER() function over a PARTITION BY clause. This technique can be applied to various scenarios involving data grouping and analysis.

Additional Considerations

There are several additional considerations when working with window functions:

• Data Type Limitations: Some data types, like integers or dates, may not work as expected in the context of row numbering.
• Partitioning: Be mindful that you can only use one PARTITION BY clause per query. If your requirements require multiple partitions, consider using a subquery.
• Window Function Order: When using multiple window functions, keep in mind the order of operations.

In addition to the provided solution, here are some variations and suggestions for further improvement:

• To select only one row with the maximum date instead of selecting all rows with RNum = 1, modify the WHERE clause as follows: SELECT * FROM ... ORDER BY data DESC LIMIT 1;

The final query becomes:

SELECT *
FROM
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY id_video_tipo ORDER BY data DESC) AS 'RNum'
    FROM `video` 
) vids
WHERE vids.RNum = 1 AND ROWNUM = 1;

This approach should give you the most recent date for each group.

• If your database system doesn’t support ROW_NUMBER() or similar functions, consider using a subquery to achieve the same result.
• For more complex queries involving multiple window functions or different aggregation methods, consider examining other SQL documentation and examples that cover these topics in depth.

Last modified on 2023-09-27