Jpql distinct

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Jpql distinct

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Which doesn't make a lot of sense - what column is the distinct based on? Is it specified on the Entity as an annotation because I couldn't find one? I agree with kazanaki 's answer, and it helped me. I wanted to select the whole entity, so I used. In my case I have many-to-many relationship, and I want to load entities with collections in one query. For this reason, the List of Post entities will contain duplicate Post entity references. QuerySyntaxException:Table is not mapped.

Learn more. Asked 10 years, 7 months ago. Active 1 year, 2 months ago. Viewed k times. Vlad Mihalcea Steve Claridge Steve Claridge 8, 8 8 gold badges 29 29 silver badges 33 33 bronze badges. Active Oldest Votes. Update: See the top-voted answer please.

My own is currently obsolete. Only kept here for historical reasons. Distinct in HQL is usually needed in Joins and not in simple examples like your own. No offense, but how could this ever be accepted as an answer? You are close. This only returns an array of that column though.

How to return whole entities with this approach? The results would be undefined. Now, you could force it with something like not sure quite how the syntax for this would work, but this should give the general idea : select c from Customer c where id in select min d.Posted on September 13, by vladmihalcea. Notice that both entities use the Fluent-style API.

We want to fetch all Post entities whose titles match a given pattern.

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However, we want to fetch the associated PostComment entities as well. As I explained in this articleyou might be tempted to use a JPA pagination query to fetch the Post entities while also join fetching the PostComment entities as illustrated by the following JPQL query:.

We want to fetch the Post entities along with their comments and limit the result set to a maximum number of entries. Notice that the SQL query uses no pagination whatsoever. As mentioned by the HHH warning message, the pagination is done in memory, which is bad. The reason why Hibernate does the pagination in memory is that it cannot just truncate the result set using SQL-level pagination. If it did that, then the result set would be truncated in the middle of the PostComments rows, therefore returning a Post entity with just a subset of comments.

Because Hibernate favors consistency, it fetches the entire result set and does the pagination in memory.

Querying JPA Entities with JPQL and Native SQL

However, that can be suboptimal, so what can we do about it? The first query will fetch the Post entity identifiers matching the provided filtering criteria. The second query will use the previously extracted Post entity identifiers to fetch the Post and the PostComment entities.

Notice the hibernate. For more details about this query hint, check out this article. As I already explained, Window Functions are the answer to many query-related problems.

The NamedNativeQuery fetches all Post entities matching the provided title along with their associated PostComment child entities. For more details about this optimization, check out this article. However, we want to transform the tabular Object[] array into a tree of parent-child entities, and for this reason, we need to use the Hibernate ResultTransformer For more details about the ResultTransformercheck out this article.

Now, not only that we can fetch both the Post and its PostComments with a single query, but we can even later modify these entities and merge them back in a subsequent read-write transaction:. If you enjoyed this article, I bet you are going to love my Book and Video Courses as well.

So, to fix the HHH issue, you have two options. Either you execute two queries and fetch the entities in either read-write or read-only mode, or you use a single query with Window Functions to fetch the entities in read-only mode. I thought the same, but actually you can not remove sorting from the first query since it would break sorting across multiple pages.

By the way, Vlad thanks a lot for your articles, it is awesome as usual. If you liked this article, you are going to love my High-Performance Java Persistence book. Why do you need all that data for? It does not fit the UI. And for data processing, you are better off using batching anyway. We are using spring for pagenation.JPQL is a powerful query language that allows you to define database queries based on your entity model. Its structure and syntax are very similar to SQL.

But there is an important difference that I want to point out before I walk you through the different parts of a JPQL query. JPQL uses the entity object model instead of database tables to define a query.

That makes it very comfortable for us Java developers, but you have to keep in mind that the database still uses SQL. It is, therefore, a good practice to activate the logging of the SQL statements during development to check the generated SQL statements.

It consists of an Author who has written one or more Publication s. A Publication can be a Book or a BlogPost. A Book might have been published by one Publisher.

The FROM clause defines from which entities the data gets selected. Hibernate, or any other JPA implementation, maps the entities to the according database tables.

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As you can see, I reference the Author entity instead of the author table and assign the identification variable a to it. The identification variable is often called alias and is similar to a variable in your Java code. It is used in all other parts of the query to reference this entity. If you want to select data from more than one entity, e.

The easiest way to do that is to use the defined associations of an entity like in the following code snippet. In this example, Hibernate uses the primary keys of the Author and Book entity to join them via the association table of the many-to-many association. JOIN s of unrelated entities are not supported by the JPA specification, but you can use a theta join which creates a cartesian product and restricts it in the WHERE clause to the records with matching foreign and primary keys.

I use this approach in the following example to join the Book with the Publisher entities. The previous examples use the defined association to join the entities. But sometimes you only want to join the related entities which fulfill additional conditions. Since JPA 2. Path expressions create implicit joins and are one of the benefits provided by the entity model.

That creates an implicit join between the Book and Publisher entity which will be translated into an additional join statement in the SQL query. When you choose an inheritance strategy that supports polymorphic queries, your query selects all instances of the specified class and its subclasses.

jpql distinct

With the model in the example for this blog post, you can, for example, select all Publication entities, which are either Book or BlogPost entities. I use that in the following code snippet to select all Author entities with their related Book entities. As you can see in the modelthe publications association defines an association between the Author and the Publication entity.

Based on my experiments, Hibernate 5. The treat operator in this example is ignored by Hibernate 5. If you need more sophisticated features for your query, you can use a native SQL query.Such Java objects are called JPA entities or just entities.

An entity is typically but not always associated with a single relational table in the underlying database, so that each entity's instance represents a certain row of that table in Java. Like relational tables, entities are typically related to each other with relationships such as one-to-one, one-to-many, many-to-one, or many-to-many. It's fairly obvious that Java applications dealing with entities require a standard mechanism to access and navigate entity instances.

How should we handle the JPQL DISTINCT keyword

The code snippets discussed in the article are taken from the Java source files used in the sample application accompanying the article. Looking through the sample archive, you may notice that this is a simple Web application based on the Java Servlet and Java Persistence API technologies. For simplicity, it doesn't use enterprise beans, issuing JPQL queries directly from within servlets.

Figure 1 illustrates the sample entities structure. As you can see, it contains a set of entities related to each other with relationships of different types. Figure 1 Relationships among the entities utilized within the sample application.

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If you have some practical experience with databases, you've most likely already got your feet wet with SQL, the standard tool offering a set of statements for accessing and manipulating information in relational databases. Both are used to access and manipulate database data, in the long run. And both use nonprocedural statements—commands recognized by a special interpreter. It's interesting to note that the EntityManager API interface offers methods that can also be used to perform retrieve, update and delete operations over entities.

In particular, these are find, merge, and remove methods. The use of those methods, however, is typically limited to a single entity instance, unless cascading takes effect, of course. In contrast, JPQL statements do not have such a limitation—you can define bulk update and delete operations over sets of entities, and define queries returning sets of entity instances. Now that you have a rough idea of how you can create and then issue a JPQL query, you might want to see some practical examples.

The following code fragment is taken from a servlet's doGet method that uses a JPQL query to obtain information about all the customers stored in the underlying relational table associated with the Customer entity specified in the query. Of special interest here are the createQuery method of the EntityManager instance and the getResultList method of the Query instance.

Spring Data JPA: Ultimate Guide to Custom Queries with @Query Annotation

As you might guess, the Query's getResultList method returns the result of a query as a List whose elements, in this particular example, are cast to type Customer. If you need to retrieve a single result, the Query API interface offers the getSingleResult method, as shown in the following example.

Note, however, that using getSingleResult will cause an exception if you get multiple results back. Also this example illustrates the use of the Query's setParameter method through which you can bind an argument to a query parameter.

With setParameter, you can bind both named and positional parameters. Here, though, you bind a named parameter. Alternatively, you might utilize the EntityManager's find method, which lets you retrieve a single entity instance based on the entity's id passed in as the parameter. In some situations, you may need to retrieve only some information from the target entity instance or instances, defining a JPQL query against a certain entity field or fields.

In JPQL, you also can use a comprised select list, selecting the data only from the entity fields of interest.

jpql distinct

In that case, however, you need to create the class to which you will cast the query result. In the following section, you will see an example of a JPQL join query whose select list is comprised of the fields derived from more than one entity. In contrast, the select list of a JPQL join query typically includes a single entity or even a single entity field. The reason for this lies in the nature of the JPA technology. Once you obtain an entity instance, you can then navigate to its related instances using corresponding getter methods.

This approach makes it unnecessary for you to define a query that will return all related entity instances of interest at once. For example, to obtain information about orders along with their line items in SQL, you would need to define a join query on both the purchaseOrders and orderLineItems tables, specifying the fields from the both tables in the select list of the query. The following snippet shows an example of JPQL join query in action.

To better understand how the entities involved are related to each other, you can turn back to Figure 1 shown in the Sample Application section earlier in the article. In the above example, you use the MAX aggregate function in the SELECT clause of the join query in order to determine the highest price product, of those that have been supplied by Tortuga Trading and have been ordered at least once.Search everywhere only in this topic.

Advanced Search. Classic List Threaded. Michael Dick. It eagerly loads b. It returns potentially multiple references to the same book. If you only wanted eager fetching of b. I have a patch that does this, and resolves the use case I described above it's in the TestLibService unit testbut I'm open to any other ideas for ways to resolve the problem.

Thanks, -mike. Pinaki Poddar. SetBackedList is indeed a good idea. But to add them to the FetchPlan. Michael Dick wrote.

Hi Pinaki, Thanks for reading through my verbose problem description and for your comments! This particular issue turned up when working with a WebSphere customer and they have been reluctant to use vendor specific options like FetchPlans in the past. I can propose FetchPlans as a solution again, but I want to have a vendor neutral answer ready if they push back. As a developer I appreciate the flexibility of FetchPlans, but I've found that using them in this manner results in a additional SQL statements.

Free forum by Nabble. Edit this page.Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please post a comment below. But if you use this clause for the first time, you might be surprised that the List of entities returned by the getResultList method contains duplicates. The SQL query joins the associated database records, and the result set consists of combinations of a parent record with each child record.

Each of these references points to the same entity object, which contains a fully initialized list of related Book entities. This is often not intended and might result in an efficient database query.

Since Hibernate 5. QueryHints and org. QueryHints class. It now only applies it to the SQL result set and returns each Author entity only once.

If you want to learn more about lazy associations or query hints, you might enjoy reading the following articles:. Your email address will not be published.

The best way to use the JPQL DISTINCT keyword with JPA and Hibernate

Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Join the free Thoughts on Java Library to get access to lots of member-only content, like a printable PDF for this post, lots of cheat sheets and 2 ebooks about Hibernate.

Join Now! Already a member? Login here. Get more recipes like this one in my new book Hibernate Tips: More than 70 solutions to common Hibernate problems. It gives you more than 70 ready-to-use recipes for topics like basic and advanced mappings, logging, Java 8 support, caching and statically and dynamically defined queries. Get it now as a paperback, ebook or PDF. Leave a Reply Cancel reply Your email address will not be published.Note that the query above might not be supported by some JPA implementations.

For example, the above query which does not use aggregates is equivalent to the following query:. The following query counts for every letter the number of countries with names that start with that letter and the number of different currencies that are used by these countries:.

jpql distinct

The query returns Object[] arrays of length 3, in which the first cell contains the initial letter as a String object, the second cell contains the number of countries in that letter's group as a Long object and the third cell contains the distinct number of currencies that are in use by countries in that group.

Other aggregate functions are applied to fields of objects in the group by using path expressions. The following query groups countries in Europe by their currency, and for each group returns the currency and the cumulative population size in countries that use that currency:.

Because grouping is performed in this query on a path expressionthis query is standard and it is expected to be supported by all JPA implementations. Only group properties which are the expressions that are used for grouping e. For example, the following query returns the sum and average population in countries that use the English language:. Therefore, in this case, only aggregate functions can be specified in the SELECT clause and individual objects and their fields become inaccessible.

The CriteriaQuery javax. See JavaDoc Reference Page The CriteriaBuilder javax. CriteriaBuilder JPA interface Used to construct criteria queries, compound selections, expressions, predicates, orderings.

One having having restriction CriteriaQuery's method Specify a restriction over the groups of the query. Expression JPA interface Type for query expressions. Predicate JPA interface The type of a simple or compound predicate: a conjunction or disjunction of restrictions. Images on this website are available under these licecnes. ObjectDB Manual. CriteriaQuery javax. Root JPA interface A root type in the from clause.

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