Sample Code Sample Application 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. 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.
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Greater or equal then: author. Is null: author. In: author. Operators for collection expressions: Is empty: author. Size: size author. Member of: :myBook member of author. You can use one or more of the operators to restrict your query result. This part of the query is very different from SQL. In SQL, you specify a set of database columns and functions as your projection. You can do the same in JPQL by selecting a set of entity attributes or functions as scalar values, but you can also define entities or constructor calls as your projection.
Hibernate, or any other JPA implementation, maps this information to a set of database columns and function calls to define the projection of the generated SQL statement. You can use any combination of them in your queries. Hibernate uses the mapping information of the selected entities to determine the database columns it has to retrieve from the database.
It then maps each row of the result set to the selected entities. But you should always keep in mind that all entities are managed by the persistence context which creates overhead for read-only use cases. Scalar values Scalar value projections are very similar to the projections you know from SQL. Instead of database columns, you select one or more entity attributes or the return value of a function call with your query.
You can see an example of it in the following code snippet. You just need to provide the fully qualified class name and specify the constructor parameters of an existing constructor. Similar to the entity projection, Hibernate generates an SQL query which returns the required database columns and uses the constructor reference to instantiate a new object for each record in the result set.
AuthorValue a. JPQL supports this operator as well. The following code snippet shows an example that uses the aggregate function count to count how often each last name occurs in the Author table. The following query selects all Author entities from the database in the ascending order of their lastName attributes.
All Authors with the same lastName are returned in descending order of their firstName. Subqueries can return one or multiple records and can use the aliases defined in the outer query. Login here. If you need one or more of them for a specific use case, you should use a native SQL query.
JPA - JPQL
The field values are extracted from or projected out of entity objects to form the query results. The results of the above query are received as a list of String values: TypedQueryjavax. See JavaDoc Reference Page Nested path expressions are also supported. Such queries are useful mainly for displaying information efficiently. They are less productive with operations that update or delete entity objects, in which managed entity objects are needed.
Java Persistence Query Language
List; import javax. EntityManager; import javax. EntityManagerFactory; import javax. Persistence; import javax. These keywords are used after Where clause in a query. Create a class named BetweenAndLikeFunctions. Query; import com.
Querying JPA Entities with JPQL and Native SQL
Greater or equal then: author. Is null: author. In: author. Operators for collection expressions: Is empty: author. Size: size author. Member of: :myBook member of author. You can use one or more of the operators to restrict your query result.