Can you explain the main differences between standard SQL, JPQL, and HQL when working with databases in Java?

The main differences between SQL, JPQL, and HQL lie in their level of abstraction and their areas of application:

• SQL (Structured Query Language) is the standard language for querying relational databases. It works directly with database tables and columns. Queries depend on the specific DBMS and operate with the physical data structure.

• JPQL (Java Persistence Query Language) is an object-oriented query language used in JPA (Java Persistence API). It operates on Java entities and their fields, not on tables and columns. JPQL is database-independent and allows writing queries at the business model level.

• HQL (Hibernate Query Language) is a query language similar to JPQL but specific to Hibernate. It also works with objects and their properties, not with tables. HQL provides extensions specific to Hibernate.

Example query:

SQL:

SELECT * FROM users WHERE age > 30;

JPQL:

SELECT u FROM User u WHERE u.age > 30

HQL is very similar to JPQL but may contain additional features specific to Hibernate.

Thus, JPQL and HQL allow working with the object model, while SQL works with the physical database structure.