I am using This JPA-Query:
SELECT DISTINCT e.label FROM Entity e
GROUP BY e.label
ORDER BY COUNT(e.label) DESC
I get no errors and the results are sorted almost correct but there are some values wrong (either two values are flipped or some single values are completly misplaced)
EDIT:
Adding COUNT(e.label) to my SELECT clause resolves this problem for this query.
But in a similar query which also contains a WHERE clause the problem persists:
SELECT DISTINCT e.label, COUNT(e.label) FROM Entity e
WHERE TYPE(e.cat) = :category
GROUP BY e.label
ORDER BY COUNT(e.label) DESC
You might need to include the COUNT(e.label) in your SELECT clause:
SELECT DISTINCT e.label, COUNT(e.label)
FROM Entity e
GROUP BY e.label
ORDER BY COUNT(e.label) DESC
UPDATE: Regarding the second query please read section 8.6. Polymorphic queries of the EntityManager documentation. It seems that if you make your queries in a way that requires multiple SELECTs, then the ORDER BY won't work anymore. Using the TYPE keyword seems to be such a case. A quote from the above link:
The following query would return all persistent objects:
from java.lang.Object o // HQL only
The interface Named might be implemented by various persistent classes:
from Named n, Named m where n.name = m.name // HQL only
Note that these last two queries will require more than one SQL SELECT. This means that the order by clause does not correctly order the whole result set. (It also means you can't call these queries using Query.scroll().)
For whatever reason the following style named query didn't work for me:
SELECT DISTINCT e.label, COUNT(e.label)
FROM Entity e
GROUP BY e.label
ORDER BY COUNT(e.label) DESC
It could be because I am using an old version of Hibernate. I got the order by working by using a number to choose the column to sort by like this:
SELECT DISTINCT e.label, COUNT(e.label)
FROM Entity e
GROUP BY e.label
ORDER BY 2 DESC
Can't see how the order could be incorrect. What is the incorrect result?
What is the SQL that is generated, if you try the same SQL directly on the database, does it give the same incorrect order?
What database are you using?
You could always sort in Java instead using sort().
Related
I have an instance of JPAQuery<?> and need to retrieve the count. However, since the table may contain many items (millions), I want to limit the count to a given maximum, say 50,000.
The current QueryDSL-Code effectively does this:
query.fetchCount();
Now my desired modifications are quite trivial in raw sql:
select count(*) from (<whatever query> limit 50000);
However, I do not know how I would express this in querydsl. The following code is not correct, because .from() takes an entity path, but query is a query:
JPAExpressions.select(Wildcard.all)
.from(query.limit(50000))
.fetchCount();
I am using querydsl 4.
JPAExpressions.select(Wildcard.all) returns a child of SimplyQuery, which you can call limit on.
JPAExpressions.select(Wildcard.all)
.from(entity)
.limit(50000)
.fetchCount();
In SQL I can do something like this:
select m.*,sq.c from message m inner join (
select max(m.id) as id, count(m.id) as c
from message m group by m.externalid
) sq on sq.id=m.id;
i.e. group the messages table, select by max(id) for each group and then return all the columns plus a count.
Can I do this in Hibernate (ideally JPQL) returning the Messages object plus the count value?
Or what's the best alternative - one query to fetch messages and then a separate one for the counts seems like it could go wrong. But I'm reluctant to have to trawl over a List if I can avoid it.
Edit:
This is as close as I can get to the equivalent Apple example from the link in the comments:
select new com.example.MessageStat(m, count(m)) from message as m group by m.externalid
But it gives this error:
ERROR [2018-08-22 11:35:54,600] org.hibernate.engine.jdbc.spi.SqlExceptionHelper: ERROR: column "message0_.id" must appear in the GROUP BY clause or be used in an aggregate function
Which I had with the SQL before adding the inner join, but I haven't been able to figure out the equivalent JPQL for that.
I have 2 entities Book and BookProperty in a one-to-many relationship. I need to retrieve books sorted by upload_date, english language books coming first using JPA Criteria with pagination and stuff (JPQL/native sql is not an option).
This is what the native query for this operation would look like (postgres):
select distinct
b.book_id,
b.name, b.author, ... ,
b.upload_date,
case when p.property_name='language' and lower(p.property_value)='english'
then 0 else 1 end as book_language,
from books b
left outer join book_properties p
on b.book_id=p.book_id
order by book_language asc, b.upload_date desc;
The problem is that I can't get the 'case' part to be selected via criteria API and sort by it. I know that it is possible via a multiselect and a tuple but I would like to avoid that, because really I do not need this column in the application. I would like to just retrieve sorted Book objects and not tuples of (Book, Integer).
I tried to move the case part into the order by and managed to compute the query via JPA Criteria, but in that case setDistinct(true) resulted in an error, because all columns in order by must be part of distinct. So moving the case part to order doesn't look like an option.
Please help me implement this query using JPA Criteria, preferably without using tuples or wrapper objects, but that will do as well if there are no other options.
I have an entity that has few fields. One of them is city name. Now I want to get list of all distinct cities from that table. How can I archive that. I tried using DISTINCT keyword, but it doesn't work.
I'm using Hibernate as JPA provider but I would like to get it in pure JPA Query.
Have you tried:
SELECT DISTINCT t.city FROM MyTable t
I'm trying to use PostgreSQL as the database for Hibernate/JPA. However, I get an error when executing a simple query. My query is as follows:
SELECT DISTINCT p FROM UserProfile p ORDER BY :order
When I execute the query, I'll pass in a value like "lastLoginDate" for :order. However, I get the following exception when trying to execute the query:
ERROR org.hibernate.util.JDBCExceptionReporter - ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
Position: 781
This query works just fine using MySQL as the database, but we'd like to move towards PostgreSQL. My overall configuration and database connection is correct, as I see the tables being created properly. Is there something I'm missing? Is there a configuration property I need to set? Any help you can give would be MUCH appreciated. Thanks!
Postgresql prohibits that query because it is ambiguous:
there's actually a definitional reason for it. Consider
SELECT DISTINCT x FROM tab ORDER BY y;
For any particular x-value in the table there might be many different y
values. Which one will you use to sort that x-value in the output?
It's not very clear what you want (an example?), but if you intend to sort all records by ':order' column, and then remove duplicated values, you can do that first with DISTINCT ON: , and then reorder:
SELECT p FROM
( SELECT DISTINCT ON (p) * from UserProfile ORDER BY p , :order)
AS UserProfileUniq
ORDER BY :order ;