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| <h3>JDOQL : Result<a id="JDOQL_:_Result"></a></h3> |
| </div> |
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| <div class="sect1"> |
| <h2 id="index">JDOQL : Result<a id="JDOQL_:_Result"></a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>As we have seen, a JDOQL query is made up of different parts. In this |
| section we look at the <em>result</em> part of the query. The result is what we |
| want returning. By default (when not specifying the result) the objects |
| returned will be of the candidate class type, where they match the query |
| filter. Firstly let’s look at what you can include in the <em>result</em> |
| clause.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><em>this</em> - the candidate instance</p> |
| </li> |
| <li> |
| <p>A field name</p> |
| </li> |
| <li> |
| <p>A variable</p> |
| </li> |
| <li> |
| <p>A parameter (though why you would want a parameter returning is hard |
| to see since you input the value in the first place)</p> |
| </li> |
| <li> |
| <p>An aggregate (count(), avg(), sum(), min(), max())</p> |
| </li> |
| <li> |
| <p>An expression involving a field (e.g "field1 + 1")</p> |
| </li> |
| <li> |
| <p>A navigational expression (navigating from one field to another …​ |
| e.g "field1.field4")</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The result is specified in JDOQL like this</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>query.setResult("count(field1), field2");</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>In <strong>Single-String JDOQL</strong> you would specify it directly.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_result_type_a_id_result_type_a">Result type<a id="Result_type"></a></h3> |
| <div class="paragraph"> |
| <p>What you specify in the <em>result</em> defines what form of result you get |
| back.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><strong>Object</strong> - this is returned if you have only a single row in the |
| results and a single column. This is achived when you specified either |
| UNIQUE, or just an aggregate (e.g "max(field2)")</p> |
| </li> |
| <li> |
| <p><strong>Object[]</strong> - this is returned if you have only a single row in the |
| results, but more than 1 column (e.g "max(field1), avg(field2)")</p> |
| </li> |
| <li> |
| <p><strong>List<Object></strong> - this is returned if you have only a single column in |
| the result, and you don’t have only aggregates in the result (e.g |
| "field2")</p> |
| </li> |
| <li> |
| <p><strong>List<Object[]></strong> - this is returned if you have more than 1 column in |
| the result, and you don’t have only aggregates in the result (e.g |
| "field2, avg(field3)")</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_aggregates_a_id_aggregates_a">Aggregates<a id="Aggregates"></a></h3> |
| <div class="paragraph"> |
| <p>There are situations when you want to return a single number for a |
| column, representing an aggregate of the values of all records. There |
| are 5 standard JDO2 aggregate functions available. These are</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><strong>avg(val)</strong> - returns the average of "val". "val" can be a field, |
| numeric field expression or "distinct field".</p> |
| </li> |
| <li> |
| <p><strong>sum(val)</strong> - returns the sum of "val". "val" can be a field, numeric |
| field expression, or "distinct field".</p> |
| </li> |
| <li> |
| <p><strong>count(val)</strong> - returns the count of records of "val". "val" can be a |
| field, or can be "this", or "distinct field".</p> |
| </li> |
| <li> |
| <p><strong>min(val)</strong> - returns the minimum of "val". "val" can be a field</p> |
| </li> |
| <li> |
| <p><strong>max(val)</strong> - returns the maximum of "val". "val" can be a field</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>So to utilise these you could specify something like</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Query q = pm.newQuery("SELECT max(price), min(price) FROM mydomain.Product WHERE status == 1");</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This will return a single row of results with 2 values, the maximum |
| price and the minimum price of all products that have status code of 1.</p> |
| </div> |
| <div class="paragraph"> |
| <p><br></p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_example_use_of_aggregates_a_id_example_use_of_aggregates_a">Example - Use of aggregates<a id="Example_-_Use_of_aggregates"></a></h3> |
| <div class="paragraph"> |
| <p>JDO 2 introduces the ability to use aggregates in queries. Here’s |
| another example using the same Product class as above, but this time |
| looking for the maximum price of products that are CD Players. Note that |
| the result for this particular query will be of type Double since there |
| is a single double precision value being returned via the "result".</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Declarative JDOQL : |
| Query query = pm.newQuery(mydomain.Product.class); |
| query.setFilter("name == \"CD Player\""); |
| query.setResult("max(this.price)"); |
| List results = (List)query.execute(); |
| Iterator iter = c.iterator(); |
| Double max_price = (Double)iter.next(); |
| |
| Single-String JDOQL : |
| Query query = pm.newQuery("SELECT max(price) FROM mydomain.Product WHERE name == \"CD Player\""); |
| List results = (List)query.execute(); |
| Iterator iter = c.iterator(); |
| Double max_price = (Double)iter.next();</pre> |
| </div> |
| </div> |
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| <p><br></p> |
| </div> |
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