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CHAPTER 10 n ADVANCED QUERIES USING CRITERIA 219 n ADVANCED QUERIES USING CRITERIA 219 Criteria crit = session.createCriteria(Supplier.class); Criteria prdCrit = crit.createCriteria(”products”); prdCrit.add(Restrictions.gt(”price”,new Double(25.0))); List results = crit.list(); Going the other way, we obtain all the products from the supplier MegaInc using many- to-one associations: Criteria crit = session.createCriteria(Product.class); Criteria suppCrit = crit.createCriteria(”supplier”); suppCrit.add(Restrictions.eq(”name”,”MegaInc”)); List results = crit.list(); Although we can use either Criteria object to obtain the results, it makes a difference which criteria we use for ordering the results. In the following example, we are ordering the supplier results by the supplier names: Criteria crit = session.createCriteria(Supplier.class); Criteria prdCrit = crit.createCriteria(”products”); prdCrit.add(Restrictions.gt(”price”,new Double(25.0))); crit.addOrder(Order.desc(”name”)); List results = prdCrit.list(); If we wanted to sort the suppliers by the descending price of their products, we would use the following line of code. This code would have to replace the previous addOrder() call on the supplier Criteria object. prdCrit.addOrder(Order.desc(”price”)); Although the products are not in the result set, SQL still allows you to order by those results. If you get mixed up with which Criteriaobject you are using and pass the wrong property name for the sort-by order, Hibernate will throw an exception. Distinct Results If you would like to work with distinct results from a criteria query, Hibernate provides a result transformer for distinct entities, org.hibernate.transform. DistinctRootEntityResultTransformer, which ensures that no duplicates will be in your query s result set. Rather than using SELECT DISTINCT with SQL, the distinct result transformer compares each of your results using their default hashCode() methods, and only adds those results with unique hash codes to your result set. This may or may not be the result you would expect from an otherwise equivalent SQL DISTINCT query, so be careful with this. An additional performance note: the comparison is done in Hibernate s Java code, not at the database, so non-unique results will still be transported across the network. Projections and Aggregates Instead of working with objects from the result set, you can treat the results from the result set as a set of rows and columns. This is similar to how you would use data from a SELECT query with JDBC; also, Hibernate supports properties, aggregate functions, and the GROUP BY clause.
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This entry was posted on Friday, July 27th, 2007 at 3:37 pm and is filed under tomcat. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.