Is there such thing CASE expression in JPQL?

Question

Let say there is a table:

TableA:Field1, Field2, Field3

and associated JPA entity class

@Entity
@Table(name="TableA")
public class TableA{
  @Id
  @Column(name="Field1")
  private Long id;

  @Column(name="Field2")
  private Long field2;

  @Column(name="Field3")
  private Long field3;

  //... more associated getter and setter...
}

Is there any way to construct a JPQL statement that loosely translated to this SQL, ie how to translated the case expression to JPQL?

select field1,
case
  when field2 = 1 then 'One'
  when field2 = 2 then 'Two'
  else 'Other number'
end,
field3
from tableA;

Answer 1

Looking over the Java Persistence API it doesn't appear to have a construct for this. It looks like you're going to need a special getter function which will do the CASE statement functionality.

Answer 2

You can use the event listeners provided by Jpa to do something when you load one row of the db, ie:

@Entity
@Table(name = "TableA")
public class TableA {

    @Id
    @Column(name = "Field1")
    private Long id;

    @Column(name = "Field2")
    private Long field2;

    @Column(name = "Field3")
    private Long field3;

    // ... more associated getter and setter...

    @Transient
    private String field4;

    @PostLoad
    private void onLoad() {
        if (field2 != null) {
            switch (field2.intValue()) {
            case 1:
                field4 = "One";
                break;
            case 2:
                field4 = "Two";
                break;
            default:
                field4 = "Other Number";
                break;
            }
        }
    }
}

(the field4 not persist in the db)

(take this like an workaround to "non implemented feature in JPA" like case statements)

Answer 3

Did this solution work for anyone? It's not working for me. I'm using Eclipselink and I get the following error: unknown state or association field [simplifiedSideString] of class [com.entity.Trade]. Trade is the entity and simplifiedSideString is the @Transient field equivalent to field4 in the above example. To the extent of my knowledge, transient fields cannot be used in JPQL queries or at least I didn't succeed in using any yet.

Answer 4

There is certainly such thing in Hibernate so when you use Hibernate as your JPA provider then you can write your query as in this example:

    Query query = entityManager.createQuery("UPDATE MNPOperationPrintDocuments o SET o.fileDownloadCount = CASE WHEN o.fileDownloadCount IS NULL THEN 1 ELSE (o.fileDownloadCount + 1) END " +
                                            " WHERE o IN (:operations)");
    query.setParameter("operations", mnpOperationPrintDocumentsList);

    int result = query.executeUpdate();