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Discover how to include `null` values in your JPA Criteria API grouping queries with a simple LEFT JOIN approach.
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Handling null Values in JPA Criteria API: Grouping with Ease

When working with databases in Java using JPA (Java Persistence API), you may encounter scenarios where you need to group results based on certain criteria while accommodating potential null values in your dataset. One common challenge is the default behavior of the Criteria API, which often omits null entries in grouped results. In this guide, we will address how to modify your JPA Criteria API queries to ensure that these null values are included in the grouping, allowing for a more comprehensive data report.

The Problem: Grouping with Null Values

Imagine you have a series of ZgloszenieEntity instances that potentially reference a user via an assignedUser relationship and an internalStatus status. When you query for results grouped by username and status, values where the internalStatus is null may simply be omitted. This could lead to a loss of valuable insights, particularly when analyzing records associated with users who have no assigned status. The situation you want to correct is illustrated by the difference between two potential outputs:

Omitting null values (current behavior):

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Including null values (desired behavior):

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The Solution: Using LEFT JOIN in JPA Criteria Queries

To achieve the inclusion of null values in your grouped results, the solution lies in leveraging the LEFT JOIN feature of the JPA Criteria API. This approach allows you to include entries in the result set even when they do not have corresponding entries in the joined entity. Here’s how you can modify your Criteria Query:

Step-by-Step Adjustments

Get the CriteriaBuilder: Start by creating an instance of the CriteriaBuilder from your EntityManager:

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Create the CriteriaQuery: Define your Criteria Query for the return type (EfficiencyStatusReportDTO):

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Define the Root Entity: Specify the root of your query, which is ZgloszenieEntity:

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Use LEFT JOINs: Change your joins to LEFT JOINs for both assignedUser and internalStatus:

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Select Fields & Grouping: Continue by selecting the necessary fields and grouping them appropriately:

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Summary

By implementing a strategy that utilizes LEFT JOIN, you open the door to include null values in your groupings within the JPA Criteria API. This adjustment not only enhances the accuracy of your reports but also ensures that no data insights are overlooked due to oversight in handling null relationships.

Feel free to apply this pattern in your own applications to improve your data handling capabilities through JPA.