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select e.name, count(elements(a)) from Event e join e.attendees a group by e.name
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The GROUP BY clause is necessary so that the count function groups the correct Events together. Like most other things in relational theory, queries returning both scalar values and values from aggregate functions have a name: vector aggregates. The query shown here is a vector aggregate. On the other hand, queries returning a single value are referred to as scalar aggregates. Scalar aggregate queries do not require a GROUP BY clause, but vector aggregate queries do. Let s look at a scalar aggregate query:
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select count(a) from Event e join e.attendees a
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Since there is nothing to group by in the SELECT clause, no GROUP BY clause is required. The GROUP BY clause can be used with the HAVING clause to place search criteria on the results of GROUP BY. Using the HAVING clause does not impact the aggregates; instead, it impacts the objects returned by the query. You can use the HAVING clause as you would a WHERE clause since the same expressions are available:
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select e.name, count(a) from Event e join e.attendees a group by e.name having length(e.name) > 10
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Criteria queries
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This query returns the Event name and number of Attendees if the Event name has more than 10 characters. You can also be more creative in the HAVING clause:
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select e.name, count(a) from Event e join e.attendees a group by e.name having size(a) > 10
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This time you re getting the Event name and Attendee count for all Events with more than 10 Attendees. Of course, now that you have your Event names, you ll probably want to order them. We ve seen the ORDER BY clause in a few queries before this point, and its usage is very straightforward. The ORDER BY clause allows you to sort the result objects in a desired order. You may sort the objects in ascending or descending order, with ascending as the default. Build on the example by adding an ORDER BY clause:
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select e.name, count(a) from Event e join e.attendees a, join e.location l group by e.name having size(a) > 10 order by e.name, l.name
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This query returns the same objects with the same criteria, only now the returned objects are sorted according to the Event and Location names. HQL provides a powerful mechanism to query persistent objects. The problem with HQL is that it is static and cannot easily be changed at runtime. Creating queries dynamically with string concatenation is a possibility, but that solution is tedious and cumbersome. Hibernate provides a simple API that can be used to create queries at runtime.
6.3 Criteria queries
The Criteria API provides an alternative method to query persistent objects. It allows you to build queries dynamically, using a simple API. Criteria queries are generally used when the number of search parameters can vary.
Querying persistent objects
Despite their relative usefulness, Criteria queries are somewhat limited. Navigating associations is cumbersome, requiring you to create another Criteria, rather than using the dot notation found in HQL. Additionally, the Criteria API does not support the equivalent of count( ), or other aggregate functions. Finally, you can only retrieve complete objects from Criteria queries. However, the Criteria API can be excellent for certain use cases for instance, in an advanced search screen where the user can select the field to search on as well as the search value. Let s look at a few examples of using Criteria queries:
Criteria criteria = session.createCriteria(Event.class); criteria.add(Restrictions.between("duration", new Integer(60), new Integer(90) ); criteria.add( Restrictions.like("name", "Presen%") ); criteria.addOrder( Order.asc("name") ); List results = criteria.list();
The Criteria query is essentially the same as the following HQL:
from Event e where (e.duration between 60 and 90) and (e.name like 'Presen%') order by e.name
The methods in the Criteria class always return the current Criteria instance, allowing you to create queries in a more concise manner:
List results = session.createCriteria(Event.class). .add( Restrictions.between("duration", new Integer(60), new Integer(90) ) .add( Restrictions.like("name", "Presen%") ) .addOrder( Order.asc("name") ) .list();
The result is the same, but the code is arguably cleaner and more concise.
Stored procedures
The Criteria API isn t as fully featured as HQL, but the ability to generate a query programmatically using a simple API can lend a great deal of power to your applications.
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