[英]Translate SQL Left Join query to EclipseLink JPA query
I am trying to write a JPA NamedQuery. 我正在尝试编写一个JPA NamedQuery。 I have a working SQL query as follows:
我有一个正常的SQL查询,如下所示:
SELECT MIN(t1.Id + 1) AS nextID
FROM
MyTable t1 LEFT JOIN MyTable t2
ON
t1.Id + 1 = t2.Id
WHERE
t2.Id IS NULL
I am not able to translate this query to JPQL syntax. 我无法将此查询转换为JPQL语法。
I think doing such a custom join is not possible with standard JPQL. 我认为使用标准JPQL无法进行这种自定义联接。 I was looking for a possibility to do it some time ago and found that Hibernate offers a proprietary extension
@JoinFormula
to achieve this, cf. 一段时间前,我一直在寻找一种实现的可能,发现Hibernate提供了专有扩展
@JoinFormula
来实现这一目标,请参见。 Hibernate @JoinFormula . 休眠@JoinFormula 。 But I couldn't find an equivalent for EclipseLink.
但是我找不到EclipseLink的等效项。
You might be able use a @NamedNativeQuery
together with an @SqlResultSetMapping
to map your SQL statement to a JPA entity, something like this: 你也许可以使用
@NamedNativeQuery
连同@SqlResultSetMapping
到您的SQL语句映射到一个JPA实体,是这样的:
@NamedNativeQuery( name = "customJoin",
query = "SELECT MIN(t1.Id + 1) AS nextID FROM MyTable t1 LEFT JOIN MyTable t2 ON t1.Id + 1 = t2.Id WHERE t2.Id IS NULL",
resultSetMapping = "customJoinMapping" )
@SqlResultSetMapping( name = "customJoinMapping",
entities = @EntityResult( entityClass = MyTable.class, fields = @FieldResult( name = "id", column = "nextID" ) ) )
@Entity
@Access( AccessType.Field )
public class MyTable {
private int id;
public int getId() {
return this.id;
}
public void setId( int id ) {
this.id = id;
}
}
UPDATE UPDATE
Much later, I found a way to customize relationship joins in EclipseLink here . 后来,我找到了一种方法来定制关系的EclipseLink加入这里 。 The feature to use is a
DescriptorCustomizer
, which can be placed on a JPA entity with @Customizer
. 使用的功能是
DescriptorCustomizer
,可以使用@Customizer
将其放置在JPA实体上。
In the customize()
method, one can express additional join criteria using the EclipseLink Expression API. 在
customize()
方法中,可以使用EclipseLink Expression API表示其他联接条件。 For example, the following code fragment would produce the additional join criteria [...] AND TargetEntity.someAttribute IN ('V', 'W', 'U')
(where someRelationship
points to a TargetEntity
). 例如,以下代码片段将产生附加的连接条件
[...] AND TargetEntity.someAttribute IN ('V', 'W', 'U')
(其中someRelationship
指向TargetEntity
)。
OneToManyMapping mapping = (OneToManyMapping) descriptor.getMappingForAttributeName( "someRelationship" );
Expression origExp = mapping.buildSelectionCriteria();
ExpressionBuilder expBuilder = origExp.getBuilder();
Expression constantExp = expBuilder.get( "someAttribute" ).in( new String[] { "V", "W", "U" } );
Expression newExp = origExp.and( constantExp );
mapping.setSelectionCriteria( newExp );
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