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joining unassociated entities in jpa (java persistence api) and hibernate can be achieved through the use of jpql (java persistence query language) or criteria api. in this tutorial, we'll cover how to perform joins between unassociated entities using both methods.

understanding the scenario

suppose we have two unassociated entities: `author` and `book`. they are not directly linked in the database schema, but we want to join them based on certain criteria (e.g., matching authors' names to book titles).

step 1: set up the entities

first, let’s define the `author` and `book` entities.

author.java



book.java



step 2: repository and entitymanager

assuming you are using spring data jpa, create a repository interface for each entity.

authorrepository.java



bookrepository.java



step 3: joining unassociated entities

using jpql

here’s how to join the entities using jpql. we can create a query to retrieve authors and books based on a common naming convention.



using criteria api

you can also achieve the same result using the criteria api, which is more type-safe and dynamic.



step 4: running the application

make sure your spring boot application is set up correctly with the necessary dependencies for jpa and hibernate. you can then run your application and call either `findauthorsandbooks()` or `findauthorsandbooksusingcriteria()` methods from a controller or service to see the results.

conclusion

in this tutorial, we defined two unassociated entities and demonstrated how to join them using both jpql and the criteria api. this approach allows you to leverage the power of jpa and hibernate for custom queries even when the entities do not have direct relationships in the database schema.

note

ensure that the database is populated with some sample data for testing.
you may need to handle transactions and exceptions appropriately in a real application setting.
the join condition used in this tutorial is just an example; you can modif ...

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JPA
Hibernate
unassociated entities
entity relationships
join entities
mapping entities
database relationships
JPA criteria API
Hibernate queries
fetch strategies
entity manager
ORM
native SQL queries
object-relational mapping
data retrieval