Retrieving the Latest Records from Multiple Categories Using SQL Queries

Retrieving 3 Latest Records from 3 Different Categories in a Database Table

When dealing with large datasets and multiple categories, retrieving the latest records for each category can be a complex task. In this article, we will explore how to achieve this using SQL queries.

Understanding the Problem

The problem statement asks us to retrieve three posts from three different categories, ordered by their last updated timestamp in descending order, and then limit the results to just those three entries. We also need to ensure that if there are ties on the timestamp, we don’t get more than three results.

Background Information

To approach this problem, we first need to understand some basic SQL concepts:

max function: This function returns the maximum value in a specified column.
group by: This clause groups rows that have the same values in one or more columns.
order by: This clause sorts the data in ascending or descending order based on one or more columns.
limit: This clause limits the number of rows returned by the query.

We also need to understand how to use joins in SQL. A join combines rows from two or more tables based on a related column between them.

Approach 1: Using Subqueries

The answer provided uses a subquery to achieve this result. Here’s a step-by-step explanation:

Step 1: Grouping and Ordering Records by Category

First, we group the records by category using GROUP BY. This ensures that all rows for each category are combined into one group.

SELECT     tst.* 
FROM       tst 
INNER JOIN (
    SELECT   category, 
             MAX(time_stamp) time_stamp,
             MAX(id) id
    FROM     tst
    GROUP BY category
    ORDER BY 2 desc
    LIMIT    3
) as topthree
ON topthree.id = tst.id;

Step 2: Ordering and Limiting Records

Next, we order the groups by their time_stamp in descending order using ORDER BY. This ensures that the most recent records are at the top.

SELECT     tst.* 
FROM       tst 
INNER JOIN (
    SELECT   category, 
             MAX(time_stamp) time_stamp,
             MAX(id) id
    FROM     tst
    GROUP BY category
    ORDER BY 2 desc
    LIMIT    3
) as topthree
ON topthree.id = tst.id;

Step 3: Joining Records

Finally, we join the topthree table with the original tst table on the id column using an INNER JOIN. This ensures that only the three most recent records for each category are returned.

SELECT     tst.* 
FROM       tst 
INNER JOIN (
    SELECT   category, 
             MAX(time_stamp) time_stamp,
             MAX(id) id
    FROM     tst
    GROUP BY category
    ORDER BY 2 desc
    LIMIT    3
) as topthree
ON topthree.id = tst.id;

Alternative Approach: Using Window Functions

Another approach to achieve this result is by using window functions, such as ROW_NUMBER() or RANK(). These functions allow us to assign a ranking to each row within a partition of a result set.

Here’s an example using ROW_NUMBER():

WITH ranked_records AS (
    SELECT category, 
           time_stamp,
           id,
           ROW_NUMBER() OVER (PARTITION BY category ORDER BY time_stamp DESC) AS rank
    FROM tst
)
SELECT * 
FROM ranked_records 
WHERE rank <= 3;

In this example, we first use a Common Table Expression (CTE) to assign a ranking to each row within each partition of the result set. We then select only the rows with a ranking less than or equal to 3.

Conclusion

Retrieving the latest records from multiple categories can be achieved using SQL queries with GROUP BY, ORDER BY, and LIMIT. We have discussed two approaches: using subqueries and window functions. Both methods provide a way to limit the results to just the three most recent entries for each category, while handling ties on the timestamp.