jpa - 使用 jpa 条件查询获取列的最大值

Question

我想获得列 relationId 的最大值来自表 ElementRelationType我已经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 都给出错误
如何获得整数最大值

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

您没有发布您收到哪些错误，所以我必须猜测。

CriteriaBuilder.max接受 Expression<N>哪里N extends Number
同时Root.get默认返回 Path<Object>不可转换为 Expression<Number> .

因此，请调用 max您需要为 root.get 指定泛型参数的工作:

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

在这里你可以替换 Number实际类型为 relationId ( Long 、 BigInteger 等)

更新: @perissf addressed您的代码的另一个问题。如果您要选择最大值(数字)，您应该声明您的 CriteriaQuery作为对 Number 的查询不是 ElementRelationTypes