{"id":3900,"date":"2020-07-01T18:23:25","date_gmt":"2020-07-01T12:53:25","guid":{"rendered":"https:\/\/www.h2kinfosys.com\/blog\/?p=3900"},"modified":"2020-07-01T18:23:27","modified_gmt":"2020-07-01T12:53:27","slug":"hibernate-query-language","status":"publish","type":"post","link":"https:\/\/www.h2kinfosys.com\/blog\/hibernate-query-language\/","title":{"rendered":"Hibernate Query Language"},"content":{"rendered":"\n

Hibernate Query Language is a Hibernate framework object-oriented query language<\/a>. It is in general similar to SQL but operates not on tables and columns, but with persistent objects and properties of persistent objects. HQL queries are translated under the hood into conventional SQL queries and that performs actions on the database.<\/p>\n\n\n\n

You can use SQL statements in the Hibernate framework, but you will need to use Native SQL. So, it can cause database portability hassles. Therefore, it is recommended to use HQL whenever possible and to take advantage of Hibernate SQL generation and caching strategies.<\/p>\n\n\n\n

In HQL keywords (SELECT, FROM, WHERE, etc.) are not case sensitive. But, table and column names are case sensitive in HQL. For example, that FrOm can be written as fRoM or just FROM. But,  edu.hibernate.test.FOO cannot be replaced with edu.hibernate.test.Foo.<\/p>\n\n\n\n

The FROM clause<\/strong><\/h3>\n\n\n\n

The FROM clause is used to load a complete persistent object into memory. Let’s look at code snippet to understand how to call the HQL query and how the simplest query with the FROM clause looks like. We will use the User class. It is valuable to mention that we do not usually need to qualify the class name with its package name because auto-import is the default. We don’t need to write “FROM edu.test.User”. Simple “FROM User” is enough.<\/p>\n\n\n\n

import<\/strong> java.util.List;\nimport<\/strong> org.hibernate.Session;\nimport<\/strong> org.hibernate.cfg.Configuration;\n\npublic<\/strong> class<\/strong> HibernateExample {\n\npublic<\/strong> static<\/strong> void<\/strong> main(String[] args) \n    {\n        Configuration configuration = new<\/strong> Configuration();\n        configuration.addAnnotatedClass(edu.hibernate.example.User.class<\/strong>);\n        Session session = configuration.configure().buildSessionFactory().openSession();    \n        String hql = \"FROM User\";\n        List results = session.createQuery(hql).getResultList();           \n    }\n}\n<\/pre>\n\n\n\n
It is possible to use aliases. For example, “from User as userEntity”. You could also write:  “from User userEntity”. In the FROM clause you can use multiple tables. In the case of multiple classes, the result will be in a cartesian product or “cross” join:<\/p>\n\n\n\n
FROM User, Car
FROM User as user, Car as car<\/pre>\n\n\n\n
The query “FROM User as user” returns instances not only of User but also of subclasses (e.g.: FacebookUser, InstaUser). HQL queries use any Java class or interface in the FROM clause. In this case, it will return the list of instances of all classes that extend that class or implement the interface. <\/p>\n\n\n\n
SELECT Clause<\/strong><\/h3>\n\n\n\n
The SELECT clause gives the developer more control over the result. With this clause, you can obtain just several properties of objects instead of the entire object. The syntax is simple. Let’s get just the first_name field from the User object. The User.firstName is a property of User object rather than a field of the User table.<\/p>\n\n\n\n
import<\/strong> java.util.List;\nimport<\/strong> org.hibernate.Session;\nimport<\/strong> org.hibernate.cfg.Configuration;\n\npublic<\/strong> class<\/strong> HibernateExample {\n\npublic<\/strong> static<\/strong> void<\/strong> main(String[] args) \n    {\n        Configuration configuration = new<\/strong> Configuration();\n        configuration.addAnnotatedClass(edu.hibernate.example.User.class<\/strong>);\n        Session session = configuration.configure().buildSessionFactory().openSession();    \n        String hql = \"SELECT U.firstName FROM User U\";\n        List results = session.createQuery(hql).getResultList();           \n    }\n}\n<\/pre>\n\n\n\n
WHERE Clause<\/strong><\/h3>\n\n\n\n
WHERE clause is used to narrow the specific objects that are returned from the database. It works the same as the SQL WHERE clause but operates with class properties rather than fields of a table. Let’s create a simple example.<\/p>\n\n\n\n
import<\/strong> java.util.List;\nimport<\/strong> org.hibernate.Session;\nimport<\/strong> org.hibernate.cfg.Configuration;\n\npublic<\/strong> class<\/strong> HibernateExample {\n\npublic<\/strong> static<\/strong> void<\/strong> main(String[] args) \n    {\n        Configuration configuration = new<\/strong> Configuration();\n        configuration.addAnnotatedClass(edu.hibernate.example.User.class<\/strong>);\n        Session session = configuration.configure().buildSessionFactory().openSession();    \n        String hql = \"FROM User U WHERE U.id = 4\";\n        List results = session.createQuery(hql).getResultList();           \n    }\n}\n<\/pre>\n\n\n\n
Expressions<\/strong><\/p>\n\n\n\n
HQL supports the list of expressions that are used in the where clause:<\/p>\n\n\n\n