Java Persistence Query Language (JPQL) is a query language that is used to execute queries against entities that are managed by the Java Persistence API (JPA). JPQL is similar to SQL in syntax, but instead of operating on tables and columns, it operates on entity classes and their attributes.

JPQL provides a powerful and flexible way to retrieve data from a database using JPA. You can use JPQL to execute queries that return a single entity, a collection of entities, or a collection of scalar values. JPQL also supports a wide range of query features such as joins, subqueries, aggregate functions, and grouping.

Some examples of JPQL queries include:

• Retrieve all entities of a specific type:

SELECT e FROM Employee e

• Retrieve entities based on a specific condition:

SELECT e FROM Employee e WHERE e.salary > 50000

• Retrieve a specific subset of data using pagination:

SELECT e FROM Employee e ORDER BY e.lastName DESC

query.setFirstResult(0);
query.setMaxResults(10);

• Perform a join between two entities:

SELECT e, d FROM Employee e JOIN e.department d WHERE d.name = 'Sales'

JPQL is a very useful tool for working with JPA, and it can help you to write complex queries in a more readable and maintainable way. To use JPQL in your Java application, you can create a query object using the EntityManager, and then execute the query and retrieve the results.