在JPQL中有这样的CASE表达吗？

假设有一张表：

TableA:Field1, Field2, Field3 

和相关的JPA实体类

 @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... } 

有没有办法构造一个松散地翻译成这个SQL的JPQL语句，即如何将case表达式翻译成JPQL？

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

Answer 1

它已在JPA 2.0中添加

用法：

 SELECT e.name, CASE WHEN (e.salary >= 100000) THEN 1 WHEN (e.salary < 100000) THEN 2 ELSE 0 END FROM Employee e 

参考： http ： //en.wikibooks.org/wiki/Java_Persistence/JPQL_BNF#New_in_JPA_2.0

Answer 2

您可以使用Jpa提供的事件侦听器在加载数据库的一行时执行某些操作，即：

 @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; } } } } 

（field4不会持久存在于db中）

（把它当作“JPA中未实现的function”的解决方法，如case语句）

Answer 3

在Hibernate中肯定存在这样的事情，所以当你使用Hibernate作为你的JPA提供者时，你可以编写你的查询，如下例所示：

  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();