如何使用JPA Criteria Builder编写带有附加计数列的查询 [英] How to write query with additional count column using JPA Criteria Builder

问题描述

我正在努力编写一个JPA查询，该查询将返回数据库中的所有P个对象，并且在它们旁边，我希望对其属性A=1的S个子对象进行计数。

SQL查询

选择p.*，(从s_table s中选择计数(s.id) 其中p.id=s.p_id和s.PropertyA=1)来自p_table p

映射：

@Entity
@Table(name = "t_table")
public class PTable{
    @Id
    private String id;

    @Version
    private Long version;

    private String subject;
   
    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    @JoinColumn(name = "p_id", referencedColumnName = "id")
    private Set<STable> sSet = new HashSet<>();
}
 
@Entity
@Table(name = "s_table")
public class STable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "p_id")
    private String pId;
  
    private String propertyA;
}

另外，您是否愿意指出任何用JPA编写复杂查询的好教程。

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<PTable> q = cb.createQuery(PTable.class);
Root<PTable> c = q.from(PTable.class);

推荐答案

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<MyPojo> cq = cb.createQuery(MyPojo.class);

Root<PTable> rootPTable = cq.from(PTable.class);
Join<PTable, STable> joinSTable = rootPTable.join(PTable_.sSet);

Subquery<Long> sqCount = cq.subquery(Long.class);
Root<STable> sqRootSTable = sqCount.from(STable.class);
Join<STable, PTable> sqJoinPTable = sqRootSTable.join(STable_.pSet);

sqCount.where(cb.and(
    cb.equal(sqJoinPTable.get(PTable_.id),rootPTable.get(PTable_.id)),
    cb.equal(sqRootSTable.get(STable_.propertyA),"1")));

sqCount.select(cb.count(sqRootSTable));

cq.multiselect(
    rootPTable.get(PTable_.id),
    rootPTable.get(PTable_.version),
    rootPTable.get(PTable_.subject),
    joinSTable.get(STable_.id),
    sqCount.getSelection(),
);

您将需要一个POJO来获取构造函数具有的结果，该结果按顺序和类型与多选参数匹配，如下所示：

public MyPojo(String pId, Long version, String subject, Long sId, Long count){
    [...]
}

您还必须更改实体以正确映射关系，通过双向和惰性操作来提高性能，如下所示：

可按键

@OneToMany(mappedBy="p",fetch = FetchType.LAZY)
private Set<STable> sSet;

稳定

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="id")
private PTable p;

这篇关于如何使用JPA Criteria Builder编写带有附加计数列的查询的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！