DISTINCT在JPA中使用哪一列，是否可以更改它？
以下是使用DISTINCT的示例JPA查询：
select DISTINCT c from Customer c
这没有多大意义 - 什么专栏是基于什么？它是否在Entity上指定为注释，因为我找不到它？
我想指定用于区分的列，例如：
select DISTINCT(c.name) c from Customer c
我正在使用MySQL和Hibernate。
答案 0 :(得分：55)
你很亲密。
select DISTINCT(c.name) from Customer c
答案 1 :(得分：13)
@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();
}
}
答案 2 :(得分：10)
更新：请参见最高投票的答案。
我自己目前已经过时了。只是出于历史原因留在这里。
在连接中通常需要HQL中的区别，而不是像您自己的简单示例中那样。
答案 3 :(得分：10)
我同意 kazanaki 的答案，这对我很有帮助。 我想选择整个实体，所以我用了
select DISTINCT(c) from Customer c
在我的情况下，我有多对多的关系，我想在一个查询中加载带有集合的实体。
我使用LEFT JOIN FETCH，最后我必须使结果不同。
答案 4 :(得分：7)
正如我在this article中所述，根据底层JPQL或Criteria API查询类型，DISTINCT在JPA中具有两种含义。
对于返回标量投影的标量查询，例如以下查询：
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);
DISTINCT关键字应该传递给基础SQL语句，因为我们希望数据库引擎在返回结果集之前过滤重复项：
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]
对于实体查询，DISTINCT具有不同的含义。
在不使用DISTINCT的情况下，查询如下：
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())
);
将要像这样联接post和post_comment表：
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]
但是对于每个关联的post行，其父post_comment记录在结果集中是重复的。因此，List个实体中的Post将包含重复的Post实体引用。
要消除Post实体引用，我们需要使用DISTINCT：
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())
);
但是随后DISTINCT也被传递给SQL查询，这是完全不希望的：
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]
通过将DISTINCT传递到SQL查询，EXECUTION PLAN将执行额外的 Sort 阶段，该阶段会增加开销而不会带来任何价值，因为父子组合始终返回唯一由于子PK列而记录：
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
要从执行计划中消除“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DISTINCT，但是Post实体引用重复项将被删除：
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
答案 5 :(得分：2)
我会使用JPA的构造函数表达式功能。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，它会是这样的。
SELECT DISTINCT new com.mypackage.MyNameType(c.name) from Customer c