Hibernate Query Language (HQL)

Hibernate Query Language (HQL) is an object-oriented query language, similar to SQL, but instead of operating on tables and columns, HQL works with persistent objects and their properties. HQL queries are translated by Hibernate into conventional SQL queries which in turns perform action on database. HQL is an SQL inspired language which allows SQL-like quiries to be writeen against persistent data objects. HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

Advantage of HQL:

• database independent
• supports polymorphic queries

Query Interface:
It is an object oriented representation of Hibernate Query. The object of Query can be obtained by calling the createQuery() method of Session interface. Keywords like SELECT, FROM and WHERE etc. are not case sensitive but properties like table and column names are case sensitive in HQL.

Examples of HQL:

• FROM Clause
  You will use FROM clause if you want to load a complete persistent objects into memory. Following is the simple syntax of using FROM clause:
  

  String hql = "FROM Employee";
  Query query = session.createQuery(hql);
  List results = query.list();

• SELECT Clause
  The SELECT clause provides more control over the result set than the from clause. If you want to obtain few properties of objects instead of the complete object, use the SELECT clause. Following is the simple syntax of using SELECT clause to get just first_name field of the Employee object: (It is notable here that Employee.firstName is a property of Employee object rather than a field of the EMPLOYEE table.)

  String hql = "SELECT E.firstName FROM Employee E";
  Query query = session.createQuery(hql);
  List results = query.list();

• WHERE Clause
  If you want to narrow the specific objects that are returned from storage, you use the WHERE clause. Following is the simple syntax of using WHERE clause:
  

  String hql = "FROM Employee E WHERE E.id = 10";
  Query query = session.createQuery(hql);
  List results = query.list(); 

More can be read at: http://www.tutorialspoint.com/hibernate/hibernate_query_language.htm