How to Retrieve Distinct Values in a Column with Total Count Using Spring JPA Pagination

Learn how to effectively use Spring JPA to get unique column values along with their counts, solving common issues like `ORDER BY` errors in pagination.
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How to Retrieve Distinct Values in a Column with Total Count Using Spring JPA Pagination

When working with data in a Spring application, there are often scenarios where developers need to retrieve distinct values from a database column along with their counts. This requirement is especially common when implementing features like filters or reports. However, sometimes developers encounter challenges, such as SQL errors related to the ORDER BY clause when using Spring Data JPA. In this post, we will explore a solution to this problem while maintaining pagination.

The Problem Statement

In a Spring JPA environment, you may attempt to run a query that collects unique values from a specific column and count them, only to find yourself facing errors like:

[[See Video to Reveal this Text or Code Snippet]]

This error message indicates that while using the DISTINCT keyword, those columns specified in the ORDER BY clause must be included in the SELECT statement as well.

The Scenario

For example, assume we want to retrieve distinct values from the column C in a table. Here's the original query causing errors:

[[See Video to Reveal this Text or Code Snippet]]

In this setup, when enabling SQL logging, it may appear that Spring automatically adds an additional column, U, to the ORDER BY clause, which is not part of the select list but corresponds to a timestamp.

The Solution

To overcome the error and streamline the process, we can simplify our query by eliminating the ORDER BY clause from the original query. Instead, we'll handle the sorting in the pagination parameters directly. Let's break it down.

Step 1: Create a Pageable with Sort

The first step is to create a Pageable object that specifies the sorting details. Here’s how you can do it:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update the Query

Remove the ORDER BY clause from the query definition. The new setup now appears as follows:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes

Removing the ORDER BY Clause: By keeping the query free of ORDER BY, we avoid the complications of the automatic addition of columns by Spring. Thus, we don't trigger the SELECT DISTINCT error.

Sorting in Pageable: The Pageable creation method enables us to manage sorting separately, ensuring that we still achieve the desired order within the fetched results efficiently.

Summary

In this article, we explored how to effectively retrieve distinct values from a database column using Spring JPA pagination while avoiding common pitfalls associated with ORDER BY clauses in SQL. By making a small adjustment to remove the clause from the query and instead incorporating it into the Pageable, we streamlined the process and improved the efficiency of our code.

If you encounter similar issues, remember that restructuring the query and utilizing the pagination features of Spring Data effectively can often solve the problem!

Feel free to share your thoughts or questions in the comments below!