How to Return a Custom Object from a Spring Data JPA GROUP BY Query?

How to Return a Custom Object from a Spring Data JPA GROUP BY Query

When performing complex queries using Spring Data JPA, it becomes necessary to return custom objects rather than the default entities. One such scenario involves grouping the results by a specific field and retrieving the count for each group. This article will explore two methods for achieving this: one for JPQL queries and one for native queries.

Solution for JPQL Queries

JPQL provides native support for returning custom objects using the new keyword. Here's how it's done:

1. Define a simple bean class to represent the desired data structure.
2. Return instances of this bean class from the repository method.

<code class="java">public interface SurveyRepository extends CrudRepository<Survey, Long> {
    @Query("SELECT new com.path.to.SurveyAnswerStatistics(v.answer, COUNT(v)) FROM Survey v GROUP BY v.answer")
    List<SurveyAnswerStatistics> findSurveyCount();
}

public class SurveyAnswerStatistics {
    private String answer;
    private Long cnt;
    // getters and setters here
}</code>

Solution for Native Queries

Native queries do not support the new keyword. Instead, use Spring Data projection interfaces:

1. Define a projection interface with the desired properties.
2. Return projected properties from the query.

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

public interface SurveyRepository extends CrudRepository<Survey, Long> {
    @Query(nativeQuery = true, value = "SELECT v.answer AS answer, COUNT(v) AS cnt FROM Survey v GROUP BY v.answer")
    List<SurveyAnswerStatistics> findSurveyCount();
}</code>

Important Notes:

• Ensure the fully-qualified path and use of the new keyword for JPQL queries.
• Use SQL's AS keyword for mapping result fields with projection interfaces in native queries.

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