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Suppose that instead of returning the entire Event object in your queries, you only want to return the name of the Event. Retrieving the entire object just to get the name is pretty inefficient. Instead, your query will only retrieve the desired data:
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select e.name from Event e
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This query returns a list of String instances containing the Event names. If you want to return the Event start date in addition to the name, add another parameter to the SELECT clause:
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select e.name, e.startDate from Event e
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Querying objects with HQL
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Each element in the returned list is an Object[] containing the specified values. The length of the Object[] array is equal to the number of columns retrieved. Listing 6.1 illustrates querying and processing multiple scalar values. Listing 6.1 Multiple scalar values
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Session session = factory.openSession(); String query = " select e.name, e.startDate from Event e "; Query query = session.createQuery("query"); List results = query.list(); for (Iterator I = results.iterator(); i.hasNext();) { Object[] row = (Object[]) i.next(); String name = (String) row[0]; Date startDate = (Date) row[1]; // }
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Looking at listing 6.1, you ll notice the values in the Object[] array are in the same order given in the query. Also, since the array contains Object instances, no primitive values can be returned from a scalar query. This limitation is also present when querying a single scalar value, since a List cannot contain primitive values.1 A common use of scalar value queries is to populate summary objects containing a subset of the data in the persistent object. In our case, a summary object would consist of the Event name and start date. When iterating over the result list, you would need to create a separate list of summary objects. Fortunately, there s a better way to do this.
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The SELECT clause can be used to create new objects, populated from values in the query. Let s look at an example:
select new EventSummary(e.name, e.startDate) from Event e
The contract for the java.util.List interface specifies that it can only store and return instances of java.lang.Object. Since primitive types (int, long, boolean, etc.) do not inherit from java.lang.Object, they cannot be stored in a java.util.List. For more information, refer to http://java.sun.com/docs/books/tutorial/collections/.
Querying persistent objects
The result list will be populated with instances of the EventSummary class. Looking at the constructor used in the HQL statement, the EventSummary class must have a constructor matching the constructor used in the HQL statement: EventSummary(String, Date). We have covered the major components of HQL queries. The following section presents the aggregate functions that are available in HQL and how they can be used in SELECT and WHERE clauses.
6.2.4 Using functions
Functions are special commands that return a computed value. In SQL, there are two types of functions: aggregate and scalar. Scalar functions typically operate on a single value and return a single value. There are also scalar functions that don t require arguments, such as now() or CURRENT_TIMESTAMP, which both return a timestamp. Aggregate functions operate on a collection of values and return a summary value. Hibernate supports five of the most commonly used SQL aggregate functions: count, avg, min, max, and sum. The functions perform the same operations as their SQL counterparts, and each operates on an expression. The expression contains the values that the function operates on. Table 6.2 summarizes the five aggregate functions. The count( ) function can also take advantage of the distinct and all keywords to filter the computed value. Let s look at some examples of using functions in HQL queries.
Table 6.2 Hibernate aggregate functions Function avg(expression) count(expression) max(expression) min(expression) sum(expression) Usage Calculates the average value of the expression. Counts the number of rows returned by the expression. Returns the maximum value in the expression. Returns the minimum value in the expression. Returns the sum of column values in the expression.
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