HQL

Hibernate Query Language, aka HQL, is the query language that Hibernate uses to make queries to the DB, its quite similar to SQL but with the particularity of using Class names or Object name instead of Tables and Fields

HQL example:

SELECT pe FROM Pelicula pe

Here pe is just and alias that references to Pelicula which is the Class that Hibernate generated when we did the Mapping. In SQL looks like this:

SQL example:

SELECT * FROM Pelicula

As you can see, in SQL we can use the asterisk to retrieve all the fields of the specified table. In HQL we use alias instead.

Hibernate has its own packages and a list of dependencies needed so it can run correctly. The following .rar file has all the necessary libraries and dependencies needed:

Transclude of lib_hibernate_7.1.rar

Hibernate includes its own classes used to make the CRUD¹ queries, such as:

• TypeQuery<{Object}>
• Query<{Object}>
• SelectionQuery<{Object}>
• MutationQuery

We are going to focus on 2 of these, SelectionQuery and MutationQuery. These 2 are quite comprehensible by just their name, SelectionQuery is used when we wanna get information from the DB, like SELECT queries, by the other hand, MutationQuery is used when we wanna make changes in the DB, like UPDATES, DELETE, INSERT queries.

SelectionQuery works by retrieving information and instancing and object of the given type. If the query returns just one result, like searching by id, its quite simple and you need to instance just one object and then write it on screen with the toString(). But, if the query returns more than 1, like for example, retrieving the movies that are ranked between a given number of stars, in this case we will use List<> to save em all in one list and read it after.

Join Types used in Hibernate: There are 2 types, Implicit JOIN and Explicit JOIN. The implicit Join uses dot . to navigate through the classes to the destiny, explicit join uses the JOIN word to navigate through tables and classes. Here are some examples:

• Implicit example: SELECT FROM Libro l WHERE l.escritor.nombre LIKE '{value}'. We are retrieving the fields of the reg Libro that has an author name that matches with the parameter.
• Explicit example: SELECT l FROM Libro l JOIN l.escritor e WHERE e.nombre LIKE '{value}' . Here same thing but with the well know Join statement.

If your program does multiple queries to the DB, the Session need to be closed ones the program finishes. Here you will find an example!

Footnotes

1. CRUD stands for: Create, Read, Update & Delete ↩