How can table joins be implemented in JPQL to obtain combined data?

In JPQL, keywords such as JOIN, LEFT JOIN, RIGHT JOIN, and FETCH JOIN are used to join tables (entities). This allows retrieving combined data from related entities.

For example, if there are entities Order and Customer related by a ManyToOne relationship, you can write a query:

SELECT o FROM Order o JOIN o.customer c WHERE c.name = :customerName

Here, JOIN o.customer c joins the orders table with the customers table based on the relationship defined in the entity. You can also use FETCH JOIN for eager loading of related data:

SELECT o FROM Order o JOIN FETCH o.customer

This reduces the number of database queries by loading related objects immediately.

Thus, joining tables in JPQL is implemented through JOINs based on relationships between entities, allowing flexible retrieval of combined data.