How to Return Custom Objects in Spring Data JPA GROUP BY Queries?

Custom Object Return in Spring Data JPA GROUP BY Queries

Spring Data JPA offers a convenient way to perform database operations using JPQL (Java Persistence Query Language). When using a custom JPQL query with a GROUP BY clause, you may want to return custom objects instead of the inbuilt SQL result arrays.

JPQL Solution

1. Define a simple bean class:

<code class="java">public class SurveyAnswerStatistics {
    private String answer;
    private Long count;
    ... // getters and setters
}</code>

2. Return bean instances from the repository method:

<code class="java">@Query("SELECT new com.path.to.SurveyAnswerStatistics(v.answer, COUNT(v)) FROM Survey v GROUP BY v.answer")
public List<SurveyAnswerStatistics> findSurveyCount();</code>

Native Query Solution

For native queries, Spring Data Projection interfaces are used instead of bean classes:

1. Define a projection interface:

<code class="java">public interface SurveyAnswerStatistics {
    String getAnswer();
    int getCnt();
    ... // additional getters
}</code>

2. Return projected properties from the query:

<code class="java">@Query(nativeQuery = true, value = "SELECT v.answer AS answer, COUNT(v) AS cnt FROM Survey v GROUP BY v.answer")
public List<SurveyAnswerStatistics> findSurveyCount();</code>

Important Notes

• Use a fully-qualified path to the bean class.
• Call the bean constructor using the new keyword.
• Pass attributes in the same order as the bean constructor parameters.
• Ensure the query is a valid JPA query.
• Use SQL AS keyword for unambiguous mapping when using native queries.

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