I got three entities:

public class BaseEntity {

  @Id
  protected BigInteger id;  

  @Column(...)
  protected int value;

  @Column(...)
  protected String name;
}

public class ChildOne{

  @Id
  protected BigInteger id;
  
  @Column(...)
  protected String oneName;
        
  @OneToOne
  @JoinColumn(name = "R_BaseEntity", nullable = false)
  protected BaseEntity baseEntity;
}

public class ChildTwo{

  @Id
  protected BigInteger id;
  
  @Column(...)
  protected String twoName;
        
  @OneToOne
  @JoinColumn(name = "R_BaseEntity", nullable = false)
  protected BaseEntity baseEntity;
}

What I finally want is a list of BaseEntity that are not already associated with ChildOne or ChildTwo. What I do:

public List<BaseEntity> get List(){
  CriteriaBuilder cb = em.getCriteriaBuilder();

    CriteriaQuery<ChildTwo> twoQuery = cb.createQuery(ChildTwo.class);
    Root<ChildTwo> twoRoot = twoQuery.from(ChildTwo.class);
    Join<ChildTwo, BaseEntity> twoJoin = twoRoot.join(ChildTwo_.baseEntity, JoinType.INNER);

    twoQuery.select(twoRoot);

    TypedQuery<ChildTwo> query = em.createQuery(twoQuery);

    List<BigInteger> baseList = new ArrayList<>();
    for (ChildTwo childTwo : query.getResultList()) {
        baseList.add(childTwo.getBaseEntity().getId());
    }

    CriteriaQuery<ChildOne> oneQuery=cb.createQuery(ChildOne.class);
    Root<ChildOne> oneRoot=oneQuery.from(ChildOne.class);
    Join<ChildOne, BaseEntity> oneJoin = oneRoot.join(ChildOne_.baseEntity,JoinType.INNER);

    oneQuery.select(oneRoot);
    TypedQuery<ChildOne> query1= em.createQuery(oneQuery);
    for(ChildOne childOne:query1.getResultList()){
        baseList.add(childOne.getBaseEntity().getId());
    }

    CriteriaQuery<BaseEntity> baseQuery=cb.createQuery(BaseEntity.class);
    Root<BaseEntity> baseRoot=baseQuery.from(BaseEntity.class);

    Predicate predicate = baseRoot.get(BaseEntity_.id).in(baseList);

    baseQuery.select(baseRoot).where(cb.not(predicate));

    return em.createQuery(baseQuery).getResultList();
}

but this is just a disgusting workaround because I can’t achive two things:

1. Get just the BaseEntity out of the join
2. transform the two joins into subqueries and do a “not in”

Any help is highly appreciated.