Solución:
Estás cerca.
select DISTINCT(c.name) from Customer c
Dependiendo del tipo de consulta JPQL o Criteria API subyacente, DISTINCT tiene dos significados en JPA.
Consultas escalares
Para consultas escalares, que devuelven una proyección escalar, como la siguiente consulta:
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);
los DISTINCT La palabra clave debe pasarse a la declaración SQL subyacente porque queremos que el motor de base de datos filtre los duplicados antes de devolver el conjunto de resultados:
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]
Consultas de entidad
Para consultas de entidades, DISTINCT tiene un significado diferente.
Sin uso DISTINCT, una consulta como la siguiente:
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())
);
se va a UNIR al post y el post_comment tablas como esta:
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]
Pero el padre post los registros se duplican en el conjunto de resultados para cada asociado post_comment hilera. Por esta razón, la List de Post las entidades contendrán duplicados Post referencias de entidad.
Para eliminar el Post referencias de entidad, necesitamos usar 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())
);
Pero entonces DISTINCT también se pasa a la consulta SQL, y eso no es deseable en absoluto:
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]
Pasando DISTINCT a la consulta SQL, el PLAN DE EJECUCIÓN ejecutará un Clasificar fase que agrega gastos generales sin aportar ningún valor, ya que las combinaciones de padres e hijos siempre devuelven registros únicos debido a la columna PK secundaria:
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
Consultas de entidad con HINT_PASS_DISTINCT_THROUGH
Para eliminar la fase de clasificación del plan de ejecución, necesitamos utilizar el HINT_PASS_DISTINCT_THROUGH Sugerencia de consulta JPA:
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())
);
Y ahora, la consulta SQL no contendrá DISTINCT pero Post Se eliminarán los duplicados de referencia de entidad:
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]
Y el Plan de ejecución confirmará que esta vez ya no tenemos una fase de clasificación adicional:
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
@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();
}
}