NHibernate - Hibernate Query Language

In this chapter, we will be covering Hibernate Query Language. HQL is shared across both Java's Hibernate and NHibernate.

  • It is the oldest query mechanism along with Criteria.

  • It was implemented very early and it is a string-based query API.

  • You access it through ISession CreateQuery, and it is almost similar to SQL.

  • It uses many of the same keywords, but has a simplified syntax.

  • It is one of the most common examples, if you're looking for how to perform a query you'll often find HQL examples.

The following is a simple example of HQL −

var customers = session.CreateQuery("select c from Customer c where c.FirstName = 'Laverne'");

  • So here you can see that they select C from customer, it looks a lot like SQL. This is an opaque string as far as NHibernate is concerned, so you don't know whether this is a valid HQL until runtime, which is one of the disadvantages.

  • One of the strengths of the LINQ provider is you can get to compile time support.

  • But HQL, is one of the most flexible query mechanisms oftenly used. It is said that, if there's no other way to do it then there's a way to do it in HQL.

Let’s have a look into a simpe example in which we will recreate our LINQ queries using HQL instead. You can get access to HQL by calling the session.CreateQuery and pass as a parameter using an HQL string.

using System; 
using System.Data; 
using System.Linq; 
using System.Reflection; 

using HibernatingRhinos.Profiler.Appender.NHibernate; 
using NHibernate.Cfg; 
using NHibernate.Criterion; 
using NHibernate.Dialect; 
using NHibernate.Driver; 
using NHibernate.Linq;

namespace NHibernateDemo {

   internal class Program { 
      
      private static void Main() { 
		
         var cfg = ConfigureNHibernate(); 
         var sessionFactory = cfg.BuildSessionFactory();
         using(var session = sessionFactory.OpenSession()) 
         
         using(var tx = session.BeginTransaction()) { 
            var customers = session.CreateQuery("select c from Customer c 
               where c.FirstName = 'Laverne'"); 
            
            foreach (var customer in customers.List<Customer>()) { 
               Console.WriteLine(customer); 
            }
				
            tx.Commit(); 
         }
			
         Console.WriteLine("Press <ENTER> to exit..."); 
         Console.ReadLine(); 
      }
		
      private static Configuration ConfigureNHibernate() { 
		
         NHibernateProfiler.Initialize(); 
         var cfg = new Configuration(); 
         
         cfg.DataBaseIntegration(x => { 
            x.ConnectionStringName = "default"; 
            x.Driver<SqlClientDriver>(); 
            x.Dialect<MsSql2008Dialect>(); 
            x.IsolationLevel = IsolationLevel.RepeatableRead; 
            x.Timeout = 10; 
            x.BatchSize = 10; 
         }); 
         
         cfg.SessionFactory().GenerateStatistics();
         cfg.AddAssembly(Assembly.GetExecutingAssembly()); 
         return cfg; 
      } 
   } 
}

  • This HQL string looks a lot like SQL, the main difference is that FirstName is the property name and not the column name.

  • So, if there's a discrepancy between the two, you use the property name. Same thing, it looks like a table name, but it's actually the name of the class that we are selecting from.

  • If the back end table was named as Customers, we would still use Customer in our HQL query.

Let’s run this application and you will see the following output.

Laverne Hegmann (4e97c816-6bce-11e1-b095-6cf049ee52be)
   Points: 74
   HasGoldStatus: True
   MemberSince: 4/4/2009 12:00:00 AM (Utc)
   CreditRating: Neutral
   AverageRating: 0

   Orders:
      Order Id: 4ea14d96-6bce-11e1-b095-6cf049ee52be
      Order Id: 4ea14d96-6bce-11e1-b096-6cf049ee52be
      Order Id: 4ea14d96-6bce-11e1-b097-6cf049ee52be
      Order Id: 4ea14d96-6bce-11e1-b098-6cf049ee52be
		
Press <ENTER> to exit...

Let’s have a look into another simple example in which we will retrieve all those customers whose FirstName starts with the letter H using HQL.

using System; 
using System.Data; 
using System.Linq; 
using System.Reflection; 

using HibernatingRhinos.Profiler.Appender.NHibernate; 
using NHibernate.Cfg; 
using NHibernate.Criterion; 
using NHibernate.Dialect; 
using NHibernate.Driver;
using NHibernate.Linq; 

namespace NHibernateDemo { 

   internal class Program {
	
      private static void Main() { 
		
         var cfg = ConfigureNHibernate(); 
         var sessionFactory = cfg.BuildSessionFactory();
         using(var session = sessionFactory.OpenSession()) 
         
         using(var tx = session.BeginTransaction()) { 
            var customers = session.CreateQuery("select c from Customer c 
               where c.FirstName like 'H%'"); 
            
            foreach (var customer in customers.List<Customer>()) { 
               Console.WriteLine(customer); 
            }
				
            tx.Commit(); 
         } 
			
         Console.WriteLine("Press <ENTER> to exit..."); 
         Console.ReadLine(); 
      } 
		
      private static Configuration ConfigureNHibernate() {
		
         NHibernateProfiler.Initialize(); 
         var cfg = new Configuration(); 
         
         cfg.DataBaseIntegration(x => { 
            x.ConnectionStringName = "default"; 
            x.Driver<SqlClientDriver>(); 
            x.Dialect<MsSql2008Dialect>(); 
            x.IsolationLevel = IsolationLevel.RepeatableRead; 
            x.Timeout = 10; 
            x.BatchSize = 10; 
         }); 
			
         cfg.SessionFactory().GenerateStatistics();
         cfg.AddAssembly(Assembly.GetExecutingAssembly()); 
         return cfg; 
      } 
   } 
}

Let’s run your application again and you will see that all of the customers whose name starts with H are returned from this query.

Herman Crooks (4ead3480-6bce-11e1-b15c-6cf049ee52be)
   Points: 74
   HasGoldStatus: True
   MemberSince: 12/3/2010 12:00:00 AM (Utc)
   CreditRating: Neutral
   AverageRating: 0

   Orders:
      Order Id: 4ead3480-6bce-11e1-b15d-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b15e-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b15f-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b160-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b161-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b162-6cf049ee52be
      Order Id: 4ead3480-6bce-11e1-b163-6cf049ee52be

Hudson Bins (4ec03f80-6bce-11e1-b2b7-6cf049ee52be)
   Points: 56
   HasGoldStatus: False
   MemberSince: 10/20/2008 12:00:00 AM (Utc)
   CreditRating: Terrible
   AverageRating: 0

   Orders:
      Order Id: 4ec03f80-6bce-11e1-b2b8-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2b9-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2ba-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2bb-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2bc-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2bd-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2be-6cf049ee52be
      Order Id: 4ec03f80-6bce-11e1-b2bf-6cf049ee52be

Hettie Feest (4ec50240-6bce-11e1-b300-6cf049ee52be)
   Points: 82
   HasGoldStatus: False
   MemberSince: 4/10/2009 12:00:00 AM (Utc)
   CreditRating: Neutral
   AverageRating: 0

   Orders:
      Order Id: 4ec50240-6bce-11e1-b301-6cf049ee52be
      Order Id: 4ec50240-6bce-11e1-b302-6cf049ee52be
      Order Id: 4ec50240-6bce-11e1-b303-6cf049ee52be
		
Press <ENTER> to exit...

We can do more complicated things like wanting all orders where customers with an order count is greater than 9. Following is the HQL query for the same.

var customers = session.CreateQuery("select c from Customer c 
   where size(c.Orders) > 9"); 
	
foreach (var customer in customers.List<Customer>()) { 
   Console.WriteLine(customer); 
}

We also need to indicate that we need a size here or count or length. In HQL, we have the option of using the special size method as shown above.

The other way to write this, if you prefer is c.Orders.size, and this has the exact effect.

var customers = session.CreateQuery("select c from Customer c 
   where c.Orders.size > 9"); 
	
foreach (var customer in customers.List<Customer>()) { 
   Console.WriteLine(customer); 
}

Let's run this application.

Lindsay Towne (4ea3aef6-6bce-11e1-b0cb-6cf049ee52be)
   Points: 50
   HasGoldStatus: False
   MemberSince: 4/13/2007 12:00:00 AM (Utc)
   CreditRating: VeryGood
   AverageRating: 0

   Orders:
      Order Id: 4ea3aef6-6bce-11e1-b0cc-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0cd-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0ce-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0cf-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d0-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d1-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d2-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d3-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d4-6cf049ee52be
      Order Id: 4ea3aef6-6bce-11e1-b0d5-6cf049ee52be

Wyman Hammes (4ea61056-6bce-11e1-b0e2-6cf049ee52be)
   Points: 32
   HasGoldStatus: False
   MemberSince: 2/5/2011 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 0

   Orders:
      Order Id: 4ea61056-6bce-11e1-b0e3-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e4-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e5-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e6-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e7-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e8-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0e9-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0ea-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0eb-6cf049ee52be
      Order Id: 4ea61056-6bce-11e1-b0ec-6cf049ee52be
		
Press <ENTER> to exit...

You can see that all the customers, who have more than 9 orders are retrieved from the database.

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