Question

我在主查询和子查询中明确定义了WHERE语句。如果省略子查询，则会在JPA日志中打印。但是，如果我添加子查询，则不应用相同的where子句。什么是JPA Criteria查询，省略了WHERE子句（选择σ）？

    // 1) MainQuery
    // Create the FROM
    Root<PubThread> rootPubThread = cq.from(PubThread.class);
    // Create the JOIN fro the first select: join-chaining. You only need the return for ordering. e.g. cq.orderBy(cb.asc(categoryJoin.get(Pub_.title)));
    Join<Pub, PubCategory> categoryJoin = rootPubThread.join(PubThread_.pups).join(Pub_.pubCategory);
    // Create the WHERE
    cq.where(criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)));
    // Create the SELECT, at last
    cq.select(rootPubThread).distinct(true);

    // 2) Subquery
    Subquery<PubThread> subquery = cq.subquery(PubThread.class); 
    Root<PubThread> rootPubThreadSub = subquery.from(PubThread.class); 
    subquery.where(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId));
    Join<Pub, PubCategory> categoryJoinSub = rootPubThreadSub.join(PubThread_.pups).join(Pub_.pubCategory);
    subquery.select(rootPubThreadSub);

    Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
    subquery.where(correlatePredicate);
    cq.where(criteriaBuilder.exists(subquery));

以下是此查询的输出：

select distinct
    pubthread0_.id as id3_,
    pubthread0_.dateCreated as dateCrea2_3_,
    pubthread0_.dateModified as dateModi3_3_,
    pubthread0_.name as name3_
from
    pubthread pubthread0_
        inner join
    pub_pubthread pups1_ ON pubthread0_.id = pups1_.pubThreads_id
        inner join
    pub pub2_ ON pups1_.pups_id = pub2_.id
        inner join
    PubCategory pubcategor3_ ON pub2_.pubCategoryId = pubcategor3_.id
where 
    exists( select 
            pubthread4_.id
        from
            pubthread pubthread4_
                inner join
            pub_pubthread pups5_ ON pubthread4_.id = pups5_.pubThreads_id
                inner join
            pub pub6_ ON pups5_.pups_id = pub6_.id
                inner join
            PubCategory pubcategor7_ ON pub6_.pubCategoryId = pubcategor7_.id
        where
             pubthread4_.id=pubthread0_.id)

Answer 1

知道了。一旦定义了谓词，where子句就会被拒绝。这是肯定的。将where子句与Predicate实例中的连接组合，并将其附加到where子句中。使用criteriaBuilder.and(..., ...)。从语义上做同样的事情。

现在，有两个连接谓词：一个用于子查询，一个用于主查询。请注意我从代码中排除的过时单select(...)（已注释）。

    CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
    CriteriaQuery<PubThread> mainQuery = criteriaBuilder
            .createQuery(PubThread.class);

    // 1) MainQuery
    // Create the FROM
    Root<PubThread> rootPubThread = mainQuery.from(PubThread.class);
    // Create the JOIN from the first select: join-chaining. You only need the return for ordering. e.g. cq.orderBy(cb.asc(categoryJoin.get(Pub_.title)));
    Join<Pub, PubCategory> categoryJoin = rootPubThread.join(PubThread_.pups).join(Pub_.pubCategory);
    // Create the WHERE
    mainQuery.where(criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)));
    // Create the SELECT, at last
    mainQuery.select(rootPubThread).distinct(true);

    // 2) Subquery
    Subquery<PubThread> subquery = mainQuery.subquery(PubThread.class); 
    Root<PubThread> rootPubThreadSub = subquery.from(PubThread.class); 
    //subquery.where(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId));
    Join<Pub, PubCategory> categoryJoinSub = rootPubThreadSub.join(PubThread_.pups).join(Pub_.pubCategory);
    subquery.select(rootPubThreadSub);

    //Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
    Predicate correlatePredicate = criteriaBuilder.and(
            criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread),
            criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)
            );
    subquery.where(correlatePredicate);     

    //Predicate correlatePredicate = criteriaBuilder.equal(rootPubThreadSub.get(PubThread_.id), rootPubThread);
    Predicate mainPredicate = criteriaBuilder.and(
            criteriaBuilder.not(criteriaBuilder.equal(rootPubThread.get(PubThread_.id), threadId)),
            criteriaBuilder.exists(subquery)
            );
    //cq.where(criteriaBuilder.exists(subquery));
    mainQuery.where(mainPredicate);


    TypedQuery<PubThread> typedQuery = em.createQuery(mainQuery);
    List<PubThread> otherPubThreads = typedQuery.getResultList();

    return otherPubThreads;

结果是这样的

    where
    pubthread4_.id=pubthread0_.id 
    and pubthread0_.id=1

为什么在使用JOINS的jpa Criteria Query中省略了where子句？

1 个答案: