使用JPA和Hibernate时,distinct如何工作

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我自己的现在已经过时了。因为历史原因才留在这里。

在连接中通常需要使用hql中的distinct,而不是在像您自己的示例中。

也见 How do you create a Distinct query in HQL

你很亲近。

select DISTINCT(c.name) from Customer c

@Entity
@NamedQuery(name = "Customer.listUniqueNames", 
            query = "SELECT DISTINCT c.name FROM Customer c")
public class Customer {
        ...

        private String name;

        public static List<String> listUniqueNames() {
             return = getEntityManager().createNamedQuery(
                   "Customer.listUniqueNames", String.class)
                   .getResultList();
        }
}

我同意 崎崎 他的回答对我有帮助。 我想选择整个实体,所以我用

 select DISTINCT(c) from Customer c

在我的例子中,我有多对多的关系,我希望在一个查询中加载带有集合的实体。

我使用了左连接获取,最后我必须使结果与众不同。

正如我在中解释的那样 this article ,取决于基础的JPQL或条件API查询类型, DISTINCT 在JPA中有两个含义。

标量查询

对于返回标量投影的标量查询,如以下查询:

List<Integer> publicationYears = entityManager
.createQuery(
    "select distinct year(p.createdOn) " +
    "from Post p " +
    "order by year(p.createdOn)", Integer.class)
.getResultList();

LOGGER.info("Publication years: {}", publicationYears);

这个 独特的 关键字应传递给基础SQL语句,因为我们希望DB引擎在返回结果集之前筛选重复项:

SELECT DISTINCT
    extract(YEAR FROM p.created_on) AS col_0_0_
FROM
    post p
ORDER BY
    extract(YEAR FROM p.created_on)

-- Publication years: [2016, 2018]

实体查询

对于实体查询, 独特的 有不同的含义。

不使用 独特的 ,类似以下查询:

List<Post> posts = entityManager
.createQuery(
    "select p " +
    "from Post p " +
    "left join fetch p.comments " +
    "where p.title = :title", Post.class)
.setParameter(
    "title", 
    "High-Performance Java Persistence eBook has been released!"
)
.getResultList();

LOGGER.info(
    "Fetched the following Post entity identifiers: {}", 
    posts.stream().map(Post::getId).collect(Collectors.toList())
);

将要加入 post 以及 post_comment 这样的表格:

SELECT p.id AS id1_0_0_,
       pc.id AS id1_1_1_,
       p.created_on AS created_2_0_0_,
       p.title AS title3_0_0_,
       pc.post_id AS post_id3_1_1_,
       pc.review AS review2_1_1_,
       pc.post_id AS post_id3_1_0__
FROM   post p
LEFT OUTER JOIN
       post_comment pc ON p.id=pc.post_id
WHERE
       p.title='High-Performance Java Persistence eBook has been released!'

-- Fetched the following Post entity identifiers: [1, 1]

但父母 邮递 每个相关联的记录在结果集中重复 后注释 行。因此, List 属于 Post 实体将包含重复的 职务 实体引用。

消除 职务 实体引用,我们需要使用 独特的 :

List<Post> posts = entityManager
.createQuery(
    "select distinct p " +
    "from Post p " +
    "left join fetch p.comments " +
    "where p.title = :title", Post.class)
.setParameter(
    "title", 
    "High-Performance Java Persistence eBook has been released!"
)
.getResultList();

LOGGER.info(
    "Fetched the following Post entity identifiers: {}", 
    posts.stream().map(Post::getId).collect(Collectors.toList())
);

但是然后 独特的 还传递给SQL查询,这一点也不可取:

SELECT DISTINCT
       p.id AS id1_0_0_,
       pc.id AS id1_1_1_,
       p.created_on AS created_2_0_0_,
       p.title AS title3_0_0_,
       pc.post_id AS post_id3_1_1_,
       pc.review AS review2_1_1_,
       pc.post_id AS post_id3_1_0__
FROM   post p
LEFT OUTER JOIN
       post_comment pc ON p.id=pc.post_id
WHERE
       p.title='High-Performance Java Persistence eBook has been released!'

-- Fetched the following Post entity identifiers: [1]

旁路 独特的 对于SQL查询,执行计划将执行一个额外的 排序 阶段,由于父子组合总是返回唯一的记录(因为子pk列),因此在不带来任何值的情况下增加了开销:

Unique  (cost=23.71..23.72 rows=1 width=1068) (actual time=0.131..0.132 rows=2 loops=1)
  ->  Sort  (cost=23.71..23.71 rows=1 width=1068) (actual time=0.131..0.131 rows=2 loops=1)
        Sort Key: p.id, pc.id, p.created_on, pc.post_id, pc.review
        Sort Method: quicksort  Memory: 25kB
        ->  Hash Right Join  (cost=11.76..23.70 rows=1 width=1068) (actual time=0.054..0.058 rows=2 loops=1)
              Hash Cond: (pc.post_id = p.id)
              ->  Seq Scan on post_comment pc  (cost=0.00..11.40 rows=140 width=532) (actual time=0.010..0.010 rows=2 loops=1)
              ->  Hash  (cost=11.75..11.75 rows=1 width=528) (actual time=0.027..0.027 rows=1 loops=1)
                    Buckets: 1024  Batches: 1  Memory Usage: 9kB
                    ->  Seq Scan on post p  (cost=0.00..11.75 rows=1 width=528) (actual time=0.017..0.018 rows=1 loops=1)
                          Filter: ((title)::text = 'High-Performance Java Persistence eBook has been released!'::text)
                          Rows Removed by Filter: 3
Planning time: 0.227 ms
Execution time: 0.179 ms

带有提示的实体查询通过

为了从执行计划中消除排序阶段,我们需要使用 HINT_PASS_DISTINCT_THROUGH JPA查询提示:

List<Post> posts = entityManager
.createQuery(
    "select distinct p " +
    "from Post p " +
    "left join fetch p.comments " +
    "where p.title = :title", Post.class)
.setParameter(
    "title", 
    "High-Performance Java Persistence eBook has been released!"
)
.setHint(QueryHints.HINT_PASS_DISTINCT_THROUGH, false)
.getResultList();

LOGGER.info(
    "Fetched the following Post entity identifiers: {}", 
    posts.stream().map(Post::getId).collect(Collectors.toList())
);

现在,SQL查询将不包含 独特的 但是 职务 将删除实体引用重复项:

SELECT
       p.id AS id1_0_0_,
       pc.id AS id1_1_1_,
       p.created_on AS created_2_0_0_,
       p.title AS title3_0_0_,
       pc.post_id AS post_id3_1_1_,
       pc.review AS review2_1_1_,
       pc.post_id AS post_id3_1_0__
FROM   post p
LEFT OUTER JOIN
       post_comment pc ON p.id=pc.post_id
WHERE
       p.title='High-Performance Java Persistence eBook has been released!'

-- Fetched the following Post entity identifiers: [1]

执行计划将确认我们这次不再有额外的排序阶段:

Hash Right Join  (cost=11.76..23.70 rows=1 width=1068) (actual time=0.066..0.069 rows=2 loops=1)
  Hash Cond: (pc.post_id = p.id)
  ->  Seq Scan on post_comment pc  (cost=0.00..11.40 rows=140 width=532) (actual time=0.011..0.011 rows=2 loops=1)
  ->  Hash  (cost=11.75..11.75 rows=1 width=528) (actual time=0.041..0.041 rows=1 loops=1)
        Buckets: 1024  Batches: 1  Memory Usage: 9kB
        ->  Seq Scan on post p  (cost=0.00..11.75 rows=1 width=528) (actual time=0.036..0.037 rows=1 loops=1)
              Filter: ((title)::text = 'High-Performance Java Persistence eBook has been released!'::text)
              Rows Removed by Filter: 3
Planning time: 1.184 ms
Execution time: 0.160 ms

我将使用JPA的构造函数表达式特性。另见以下答案:

JPQL Constructor Expression - org.hibernate.hql.ast.QuerySyntaxException:Table is not mapped

按照问题中的例子,应该是这样的。

SELECT DISTINCT new com.mypackage.MyNameType(c.name) from Customer c