使用jpa条件查询获取列的最大值

Question

我想relationId从表中获取列的最大值ElementRelationType 我已经编写了代码,但它给出了错误

CriteriaBuilder cb1 = entityManager.getCriteriaBuilder();
CriteriaQuery<ElementRelationTypes> cq1 = cb1.createQuery(ElementRelationTypes.class);
Root<ElementRelationTypes> root = cq1.from(ElementRelationTypes.class);
cq1.select(cb1.max(root.get("relationId")));

select和max都给出了如何获得整数最大值的错误

public class ElementRelationTypes {

private RelationId relationLangPK=new RelationId(); 

private Country country;
private Status status;


@EmbeddedId
public RelationId getRelationLangPK() {
    return relationLangPK;
}
public void setRelationLangPK(RelationId relationLangPK) {
    this.relationLangPK = relationLangPK;
}

@Transient
public Integer getRelationId() {
    return getRelationLangPK().getRelationId();
}
public void setRelationId(Integer relationId) {
    getRelationLangPK().setRelationId(relationId);
}
@Transient
public Language getLanguage() {
    return getRelationLangPK().getLanguage();
}
public void setLanguageCode(Language language) {
    getRelationLangPK().setLanguage(language);
}

和

public class RelationId implements Serializable {
private static final long serialVersionUID = 1L;
private Integer relationId;
private Language language;

@JoinColumn(name=PersistenseConstants.ELEMENT_RELATION_TYPE_COL_RELATION_ID)
public Integer getRelationId() {
    return relationId;
}

public void setRelationId(Integer relationId) {
    this.relationId = relationId;
}

@OneToOne
@JoinColumn(name=PersistenseConstants.LANGUAGE_ENTITY_COL_LANG_CODE)
public Language getLanguage() {
    return language;
}

public void setLanguage(Language language) {
    this.language = language;
}

Answer 1

你没有发布你收到的错误,所以我不得不猜.

CriteriaBuilder.max接受Expression<N>在哪里N extends Number

同时Root.get默认返回Path<Object>哪个是不可转换的Expression<Number>.

因此,要使您的max工作调用,您需要将泛型参数指定为root.get:

cq1.select(cb1.max(root.<Number>get("relationId")));

在这里你可以替换Number与实际类型的relationId(Long,BigInteger等等)

更新: @perissf 解决了您的代码的另一个问题.如果要选择最大值(数字),则应将您CriteriaQuery的查询声明为Number不是ElementRelationTypes